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Mouse fix for high-DPI non-macOS systems (e.g. Wayland)

The "DPI hack" for Apple Retina systems is actually necessary for all
high-DPI systems. Enable it unconditionally — it's harmless on
non-high-DPI systems — and clean it up a little. In particular:
- Use SDL_GetRendererOutputSize() instead of SDL_GL_GetDrawableSize(),
  which is not guaranteed to work on non-OpenGL-based renderers.
- Move the display-mode related stuff behind a video.useDesktopMouseCoords
  check: ideally we should be using SDL_GetWindowSize() instead.
- Update the comments to describe the DPI calculation more fully.

Note that, in the soon-to-be-released SDL 2.0.18, the helper function
SDL_RenderWindowToLogical() could probably replace a lot of this.
(Similarly, the use of SDL_SetWindowMouseRect() will make more sense
instead of warping for the software cursor case), but those should wait
until after 2.0.18 is fully released and available.

--- a/src/ft2_video.c

+++ b/src/ft2_video.c

@@ -221,15 +221,22 @@

 static void updateRenderSizeVars(void)

 {

 	float fXScale, fYScale;

-	SDL_DisplayMode dm;

-	int32_t di = SDL_GetWindowDisplayIndex(video.window);

-	if (di < 0)

-		di = 0; // return display index 0 (default) on error

+	if (video.useDesktopMouseCoords)

+	{

+		SDL_DisplayMode dm;

+		int32_t di = SDL_GetWindowDisplayIndex(video.window);

+		if (di < 0)

+			di = 0; // return display index 0 (default) on error

-	SDL_GetDesktopDisplayMode(di, &dm);

-	video.displayW = dm.w;

-	video.displayH = dm.h;

+		SDL_GetDesktopDisplayMode(di, &dm);

+		video.displayW = dm.w;

+		video.displayH = dm.h;

+	}

+	else

+	{

+		SDL_GetWindowSize(video.window, &video.displayW, &video.displayH);

+	}

 	if (video.fullscreen)

 	{

@@ -247,10 +254,15 @@

 			video.renderW = (int32_t)(SCREEN_W * fXScale);

 			video.renderH = (int32_t)(SCREEN_H * fYScale);

-			// retina high-DPI hackery (SDL2 is bad at reporting actual rendering sizes on macOS w/ high-DPI)

-#ifdef __APPLE__

+			// high-DPI hackery:

+			//  On high-DPI systems, the display w/h are given in logical pixels,

+			//  but the renderer size is given in physical pixels. Since our internal

+			//  render{X,Y,W,H} variables need to be in logical coordinates, as that's

+			//  what mouse input uses, scale them by the screen's DPI scale factor,

+			//  which is the physical (renderer) size / the logical (window) size.

+			//  On non high-DPI systems, this is effectively a no-op.

 			int32_t actualScreenW, actualScreenH;

-			SDL_GL_GetDrawableSize(video.window, &actualScreenW, &actualScreenH);

+			SDL_GetRendererOutputSize(video.renderer, &actualScreenW, &actualScreenH);

 			const double dXUpscale = (const double)actualScreenW / video.displayW;

 			const double dYUpscale = (const double)actualScreenH / video.displayH;

@@ -258,7 +270,7 @@

 			// downscale back to correct sizes

 			if (dXUpscale != 0.0) video.renderW = (int32_t)(video.renderW / dXUpscale);

 			if (dYUpscale != 0.0) video.renderH = (int32_t)(video.renderH / dYUpscale);

-#endif

+

 			video.renderX = (video.displayW - video.renderW) >> 1;

 			video.renderY = (video.displayH - video.renderH) >> 1;

 		}