Added an EFB peek cache to the GL video plugin

Most of the games using EFB peeks are suffering from major performance problems
when these peeks are not disabled in the graphics settings. This is an attempt
to fix this in the GL renderer by doing the glReadPixels in bulk: instead of
doing a lot of 1x1 pixel reads, read for 64x64 pixels at once and keep that in
a cache.

Deck menu in Baten Kaitos: 3FPS -> 54FPS
Character creation in Monster Hunter Tri: 7FPS -> 60FPS

Source/Plugins/Plugin_VideoOGL/Src/VertexManager.cpp

@@ -268,6 +268,8 @@ void VertexManager::vFlush()
 #endif
 	g_Config.iSaveTargetId++;
 
+	ClearEFBCache();
+
 	GL_REPORT_ERRORD();
 }