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OpenGL fixes and optimisations

Now the VBO isn't resized constantly, and CreateSurface preserves
the currently-bound texture.

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

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

@@ -80,7 +80,8 @@

 static GLuint vertex_buffer_id;

 static GLuint framebuffer_id;

-static VertexBufferSlot *vertex_buffer;

+static VertexBufferSlot *local_vertex_buffer;

+static unsigned long local_vertex_buffer_size;

 static unsigned long current_vertex_buffer_slot;

 static RenderMode last_render_mode;

@@ -342,26 +343,33 @@

 static VertexBufferSlot* GetVertexBufferSlot(void)

 {

-	static unsigned long max_slots = 0;

-

-	if (current_vertex_buffer_slot >= max_slots)

+	if (current_vertex_buffer_slot >= local_vertex_buffer_size)

 	{

-		if (max_slots == 0)

-			max_slots = 1;

+		if (local_vertex_buffer_size == 0)

+			local_vertex_buffer_size = 1;

 		else

-			max_slots <<= 1;

+			local_vertex_buffer_size <<= 1;

-		vertex_buffer = (VertexBufferSlot*)realloc(vertex_buffer, max_slots * sizeof(VertexBufferSlot));

-		glBufferData(GL_ARRAY_BUFFER, max_slots * sizeof(VertexBufferSlot), NULL, GL_STREAM_DRAW);

+		local_vertex_buffer = (VertexBufferSlot*)realloc(local_vertex_buffer, local_vertex_buffer_size * sizeof(VertexBufferSlot));

 	}

-	return &vertex_buffer[current_vertex_buffer_slot++];

+	return &local_vertex_buffer[current_vertex_buffer_slot++];

 }

 static void FlushVertexBuffer(void)

 {

-	glBufferSubData(GL_ARRAY_BUFFER, 0, sizeof(VertexBufferSlot) * current_vertex_buffer_slot, vertex_buffer);

+	static unsigned long vertex_buffer_size = 0;

+	if (local_vertex_buffer_size > vertex_buffer_size)

+	{

+		vertex_buffer_size = local_vertex_buffer_size;

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

+	}

+	else

+	{

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

+	}

+

 	if (last_render_mode == MODE_DRAW_GLYPH_LCD)

 	{

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

@@ -532,7 +540,7 @@

 void Backend_Deinit(void)

 {

-	free(vertex_buffer);

+	free(local_vertex_buffer);

 	glDeleteTextures(1, &framebuffer.texture_id);

 	glDeleteFramebuffers(1, &framebuffer_id);

@@ -618,6 +626,9 @@

 	if (surface == NULL)

 		return NULL;

+	GLint previously_bound_texture;

+	glGetIntegerv(GL_TEXTURE_BINDING_2D, &previously_bound_texture);

+

 	glGenTextures(1, &surface->texture_id);

 	glBindTexture(GL_TEXTURE_2D, surface->texture_id);

 #ifdef USE_OPENGLES2

@@ -633,6 +644,8 @@

 	glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_R, GL_CLAMP_TO_EDGE);

 #endif

+	glBindTexture(GL_TEXTURE_2D, previously_bound_texture);

+

 	surface->width = width;

 	surface->height = height;

@@ -797,8 +810,6 @@

 		glDisableVertexAttribArray(2);

 		glUniform4f(program_colour_fill_uniform_colour, red / 255.0f, green / 255.0f, blue / 255.0f, 1.0f);

-

-		current_vertex_buffer_slot = 0;

 	}

 	const GLfloat vertex_left = (rect->left * (2.0f / surface->width)) - 1.0f;