Define extensions DRI_MutableRenderBufferDriver and
DRI_MutableRenderBufferLoader. These are the two halves for
EGL_KHR_mutable_render_buffer.
Outside the DRI code there is one additional change. Add
gl_config::mutableRenderBuffer to match
__DRI_ATTRIB_MUTABLE_RENDER_BUFFER. Neither are used yet.
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>