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Double-buffer the OpenGL VBO

This should reduce stalling when the OpenGL driver is still
processing the buffer when we're about to upload to it.

Hopefully, this is what was making the OpenGL ES 2.0 renderer so much
slower than the SDLTexture renderer on the Raspberry Pi 3B (SDL uses
*8* buffers). Unfortunately, I don't have access to it right now, so
I can't test this.

--- a/src/Backends/Rendering/OpenGL3.cpp

+++ b/src/Backends/Rendering/OpenGL3.cpp

@@ -17,6 +17,8 @@

 #include "../../WindowsWrapper.h"

+#define TOTAL_VBOS 2

+

 typedef enum RenderMode

 {

 	MODE_BLANK,

@@ -77,7 +79,7 @@

 #ifndef USE_OPENGLES2

 static GLuint vertex_array_id;

 #endif

-static GLuint vertex_buffer_id;

+static GLuint vertex_buffer_ids[TOTAL_VBOS];

 static GLuint framebuffer_id;

 static VertexBufferSlot *local_vertex_buffer;

@@ -358,12 +360,17 @@

 static void FlushVertexBuffer(void)

 {

-	static unsigned long vertex_buffer_size = 0;

+	static unsigned long vertex_buffer_size[TOTAL_VBOS];

+	static unsigned int current_vertex_buffer = 0;

-	if (local_vertex_buffer_size > vertex_buffer_size)

+	glBindBuffer(GL_ARRAY_BUFFER, vertex_buffer_ids[current_vertex_buffer]);

+	glVertexAttribPointer(1, 2, GL_FLOAT, GL_FALSE, sizeof(Vertex), (GLvoid*)offsetof(Vertex, vertex_coordinate));

+	glVertexAttribPointer(2, 2, GL_FLOAT, GL_FALSE, sizeof(Vertex), (GLvoid*)offsetof(Vertex, texture_coordinate));

+

+	if (local_vertex_buffer_size > vertex_buffer_size[current_vertex_buffer])

 	{

-		vertex_buffer_size = local_vertex_buffer_size;

-		glBufferData(GL_ARRAY_BUFFER, vertex_buffer_size * sizeof(VertexBufferSlot), local_vertex_buffer, GL_STREAM_DRAW);

+		vertex_buffer_size[current_vertex_buffer] = local_vertex_buffer_size;

+		glBufferData(GL_ARRAY_BUFFER, vertex_buffer_size[current_vertex_buffer] * sizeof(VertexBufferSlot), local_vertex_buffer, GL_STREAM_DRAW);

 	}

 	else

 	{

@@ -370,6 +377,9 @@

 		glBufferSubData(GL_ARRAY_BUFFER, 0, current_vertex_buffer_slot * sizeof(VertexBufferSlot), local_vertex_buffer);

 	}

+	if (++current_vertex_buffer >= TOTAL_VBOS)

+		current_vertex_buffer = 0;

+

 	if (last_render_mode == MODE_DRAW_GLYPH_LCD)

 	{

 		// Here we're going to draw with per-component alpha.

@@ -453,14 +463,11 @@

 					glBindVertexArray(vertex_array_id);

 				#endif

-					// Set up Vertex Buffer Object

-					glGenBuffers(1, &vertex_buffer_id);

-					glBindBuffer(GL_ARRAY_BUFFER, vertex_buffer_id);

+					// Set up Vertex Buffer Objects

+					glGenBuffers(TOTAL_VBOS, vertex_buffer_ids);

 					// Set up the vertex attributes

 					glEnableVertexAttribArray(1);

-					glVertexAttribPointer(1, 2, GL_FLOAT, GL_FALSE, sizeof(Vertex), (GLvoid*)offsetof(Vertex, vertex_coordinate));

-					glVertexAttribPointer(2, 2, GL_FLOAT, GL_FALSE, sizeof(Vertex), (GLvoid*)offsetof(Vertex, texture_coordinate));

 					// Set up our shaders

 					program_texture = CompileShader(vertex_shader_texture, fragment_shader_texture);

@@ -524,7 +531,7 @@

 					if (program_texture != 0)

 						glDeleteProgram(program_texture);

-					glDeleteBuffers(1, &vertex_buffer_id);

+					glDeleteBuffers(TOTAL_VBOS, vertex_buffer_ids);

 				#ifndef USE_OPENGLES2

 					glDeleteVertexArrays(1, &vertex_array_id);

 				#endif

@@ -550,7 +557,7 @@

 	glDeleteProgram(program_colour_fill);

 	glDeleteProgram(program_texture_colour_key);

 	glDeleteProgram(program_texture);

-	glDeleteBuffers(1, &vertex_buffer_id);

+	glDeleteBuffers(TOTAL_VBOS, vertex_buffer_ids);

 #ifndef USE_OPENGLES2

 	glDeleteVertexArrays(1, &vertex_array_id);

 #endif

-- 

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