dri: Add createContext hook to __DRI_MESA.

Again, reduces switching on driver type in the loader.

Reviewed-by: Adam Jackson <ajax@redhat.com>
Acked-by: Eric Engestrom <eric@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20069>

include/GL/internal/mesa_interface.h

@@ -57,6 +57,8 @@ struct __DRImesaCoreExtensionRec {
     */
    __DRIcreateNewScreen2Func createNewScreen;
 
+   __DRIcreateContextAttribsFunc createContext;
+
    /* driver function for finishing initialization inside createNewScreen(). */
    const __DRIconfig **(*initScreen)(struct dri_screen *screen);
 };