Check for low GPU memory and try to disable cache.

This may help with phones and even desktops without much VRAM/RAM.
Normally OpenGL will copy between them, though, so it should not happen
often, especially not on desktops.

GPU/GLES/TextureCache.h

@@ -108,6 +108,7 @@ private:
 	TexCache secondCache;
 
 	bool clearCacheNextFrame_;
+	bool lowMemoryMode_;
 	TextureScaler scaler;
 
 	SimpleBuf<u32> tmpTexBuf32;