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Call glFramebufferTexture2D as rarely as possible

Turns out performance is absolutely abysmal on my laptop's copy of
Windows 10 (AMD A9 APU).

This is only one of the weird bottleecks: glFramebufferTexture2D
is a CPU sinkhole, so don't call it often.

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

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

@@ -205,6 +205,17 @@

 	return program_id;

 }

+static void SetFramebufferTarget(GLuint texture_id)

+{

+	static GLuint last_framebuffer_target;

+

+	if (texture_id != last_framebuffer_target)

+	{

+		last_framebuffer_target = texture_id;

+		glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, texture_id, 0);

+	}

+}

+

 SDL_Window* Backend_CreateWindow(const char *title, int width, int height)

 {

 	SDL_GL_SetAttribute(SDL_GL_CONTEXT_PROFILE_MASK, SDL_GL_CONTEXT_PROFILE_CORE);

@@ -410,7 +421,7 @@

 		return;

 	// Point our framebuffer to the destination texture

-	glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, destination_surface->texture_id, 0);

+	SetFramebufferTarget(destination_surface->texture_id);

 	glViewport(0, 0, destination_surface->width, destination_surface->height);

 	// Switch to colour-key shader if we have to

@@ -474,7 +485,7 @@

 		return;

 	// Point our framebuffer to the destination texture

-	glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, surface->texture_id, 0);

+	SetFramebufferTarget(surface->texture_id);

 	glViewport(0, 0, surface->width, surface->height);

 	glUseProgram(program_colour_fill);

@@ -597,7 +608,7 @@

 		return;

 	// Point our framebuffer to the destination texture

-	glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, surface->texture_id, 0);

+	SetFramebufferTarget(surface->texture_id);

 	glViewport(0, 0, surface->width, surface->height);

 	glEnable(GL_BLEND);