Rework the GL_READ_BUFFER, GL_DRAW_BUFFER state repairs that Roland previously did.

Rework the GL_READ_BUFFER, GL_DRAW_BUFFER state repairs that Roland previously did.

Basically, in update_framebuffer() (which should be called after an FBO is bound
with MakeCurrent or BindFramebuffer) we check if the FBO is a window-system FBO.
If it is, update the FBO's GL_READ/DRAW_BUFFER state according to the context state.
Old code still in place but disabled with #if 0 / #endif.