mesa: throw a log warning any time a fallback texture is used

this generally means an app is broken in some way, so throw a log
message to be helpful

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16821>

src/mesa/main/texstate.c

@@ -704,6 +704,7 @@ update_single_program_texture(struct gl_context *ctx, struct gl_program *prog,
     * Mesa implements this by creating a hidden texture object with a pixel of
     * that value.
     */
+   _mesa_log("MESA: Using fallback texture for target %u\n", target_index);
    texObj = _mesa_get_fallback_texture(ctx, target_index);
    assert(texObj);