Bug 743830: Added 'gfx.xrender.enableded' pref (true by default). The pref is fetched in gfPlatformGtk.cpp and can be accessed through gfxPlatformGtk::UseXRender(). UseXRender is used in GLContextProviderGLX to set mUseTextureFromPixmap. r=jrmuizel,gw280