Bug 1097877. Move GLBlitTextureImageHelper from gfx/gl to gfx/layers/opengl. r=nical

The only user is in layers/opengl. This is needed to do better state tracking
in the compositor.

--HG--
rename : gfx/gl/GLBlitTextureImageHelper.cpp => gfx/layers/opengl/GLBlitTextureImageHelper.cpp
rename : gfx/gl/GLBlitTextureImageHelper.h => gfx/layers/opengl/GLBlitTextureImageHelper.h
extra : rebase_source : bba00cbfac9253ca933721659dadbda2bfbe65c5