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Enable window-resizing in the SDLTexture renderer

Previously, the window was a fixed size just like the original.

This change highlights a weird issue in CSE2: this doesn't exactly
match the behaviour of the original game, so why did I change it?
Simple: monitors had much lower pixel-densities back in the early
2000s, meaning that 320x240 and 640x480 weren't as laughably small as
they are today.

I like to think of CSE2's portable branch as Cave Story's equivalent
to Chocolate Doom: the point isn't to replicate the game's behaviour
100% - that's the accurate branch's job - no, the point of the
portable branch is to replicate the *experience* of the game, as it
was back in 2004. This means no commically-small windows.

This is the same reason font anti-aliasing is disabled, even in
versions of Windows later than XP.

Sidenote: the OpenGL3/OpenGLES2 renderers already had this feature,
but they used linear-filtering, causing the screen to be extremely
blurry in 320x240 mode. This renderer uses a better method, which
does apply slight blurring at the edges of pixels at resolutions
that aren't an exact multiple of 320x240/640x480, but otherwise it
resembles nearest-neighbour. This is way nicer to look at, and fits
the game's aesthetic. This feature will be ported to the other
renderers soon.

--- a/src/Backends/Rendering/SDLTexture.cpp

+++ b/src/Backends/Rendering/SDLTexture.cpp

@@ -12,6 +12,9 @@

 #include "../Misc.h"

 #include "../Shared/SDL2.h"

+#define MIN(a,b) ((a) < (b) ? (a) : (b))

+#define MAX(a,b) ((a) > (b) ? (a) : (b))

+

 typedef struct RenderBackend_Surface

 {

 	SDL_Texture *texture;

@@ -34,7 +37,10 @@

 static SDL_Renderer *renderer;

 static RenderBackend_Surface framebuffer;

+static RenderBackend_Surface upscaled_framebuffer;

+static SDL_Rect upscaled_framebuffer_rect;

+

 static RenderBackend_Surface *surface_list_head;

 static void RectToSDLRect(const RenderBackend_Rect *rect, SDL_Rect *sdl_rect)

@@ -64,7 +70,7 @@

 			Backend_PrintInfo("%s", info.name);

 	}

-	window = SDL_CreateWindow(window_title, SDL_WINDOWPOS_CENTERED, SDL_WINDOWPOS_CENTERED, screen_width, screen_height, 0);

+	window = SDL_CreateWindow(window_title, SDL_WINDOWPOS_CENTERED, SDL_WINDOWPOS_CENTERED, screen_width, screen_height, SDL_WINDOW_RESIZABLE);

 	if (window != NULL)

 	{

@@ -93,6 +99,8 @@

 				framebuffer.width = screen_width;

 				framebuffer.height = screen_height;

+				RenderBackend_HandleWindowResize(screen_width, screen_height);

+

 				Backend_PostWindowCreation();

 				return &framebuffer;

@@ -132,12 +140,23 @@

 void RenderBackend_DrawScreen(void)

 {

+	if (SDL_SetRenderTarget(renderer, upscaled_framebuffer.texture) < 0)

+		Backend_PrintError("Couldn't set upscaled framebuffer as the current rendering target: %s", SDL_GetError());

+

+	if (SDL_RenderCopy(renderer, framebuffer.texture, NULL, NULL) < 0)

+		Backend_PrintError("Failed to copy framebuffer texture to upscaled framebuffer: %s", SDL_GetError());

+

 	if (SDL_SetRenderTarget(renderer, NULL) < 0)

 		Backend_PrintError("Couldn't set default render target as the current rendering target: %s", SDL_GetError());

-	if (SDL_RenderCopy(renderer, framebuffer.texture, NULL, NULL) < 0)

-		Backend_PrintError("Failed to copy framebuffer texture to default render target: %s", SDL_GetError());

+	if (SDL_SetRenderDrawColor(renderer, 0, 0, 0, 0xFF) < 0)

+		Backend_PrintError("Couldn't set color for drawing operations: %s", SDL_GetError());

+	SDL_RenderClear(renderer);

+

+	if (SDL_RenderCopy(renderer, upscaled_framebuffer.texture, NULL, &upscaled_framebuffer_rect) < 0)

+		Backend_PrintError("Failed to copy upscaled framebuffer texture to default render target: %s", SDL_GetError());

+

 	SDL_RenderPresent(renderer);

 }

@@ -418,8 +437,36 @@

 void RenderBackend_HandleWindowResize(size_t width, size_t height)

 {

-	(void)width;

-	(void)height;

+	size_t upscale_factor = MAX(1, MIN((width + framebuffer.width / 2) / framebuffer.width, (height + framebuffer.height / 2) / framebuffer.height));

-	// No problem for us

+	upscaled_framebuffer.width = framebuffer.width * upscale_factor;

+	upscaled_framebuffer.height = framebuffer.height * upscale_factor;

+

+	if (upscaled_framebuffer.texture != NULL)

+		SDL_DestroyTexture(upscaled_framebuffer.texture);

+

+	SDL_SetHint(SDL_HINT_RENDER_SCALE_QUALITY, "linear");

+	upscaled_framebuffer.texture = SDL_CreateTexture(renderer, SDL_PIXELFORMAT_RGBA32, SDL_TEXTUREACCESS_TARGET, upscaled_framebuffer.width, upscaled_framebuffer.height * upscale_factor);

+	SDL_SetHint(SDL_HINT_RENDER_SCALE_QUALITY, "nearest");

+

+	if (upscaled_framebuffer.texture == NULL)

+		Backend_PrintError("Couldn't regenerate upscaled framebuffer");

+

+	// Create rect that forces 4:3 no matter what size the window is

+	float window_ratio = (float)width / height;

+	float framebuffer_ratio = (float)upscaled_framebuffer.width / upscaled_framebuffer.height;

+

+	if (window_ratio >= framebuffer_ratio)

+	{

+		upscaled_framebuffer_rect.w = height * framebuffer_ratio;

+		upscaled_framebuffer_rect.h = height;

+	}

+	else

+	{

+		upscaled_framebuffer_rect.w = width;

+		upscaled_framebuffer_rect.h = width / framebuffer_ratio;

+	}

+

+	upscaled_framebuffer_rect.x = (width - upscaled_framebuffer_rect.w) / 2;

+	upscaled_framebuffer_rect.y = (height - upscaled_framebuffer_rect.h) / 2;

 }