Removed generating new texture repeatedly during character cache.
Copy valid region only from old texture after ReallocTexture().
As a result, reduced stuttering video playback with subtitle.
Merge remote-tracking branch 'origin/opt_ttf_gl' into opt_ttf_gl

Conflicts:
	xbmc/guilib/GUIFontTTFGL.cpp
	xbmc/guilib/GUIFontTTFGL.h