private api, but useful for 2d gui code that suddenly finds itself
needing to get a set of textures onto some fbo
I didn't want to include ARB_copy_image since it looks like its from
texture/renderbuffer -> texture/renderbuffer while this class implies
texture -> write fbo. We could wrap ARB_copy_image in QOpenGLTexture or
some other class or we can add it later.
I have not added any QOpenGLTexture functions since this class opperates
on the GLuint identifier. We can add overloads later.
Change-Id: I3e565b33466c1c183a249a33c3e82c6786debd55
Reviewed-by: Paul Olav Tvete <paul.tvete@digia.com>
