glamor: Dump backtrace on GL error

Currrently, when a GL error is triggered, glamor would log the error
which may not be sufficient to trace it back to the cause of the error.

Also dump the backtrace which may give more information as to where the
error comes from.

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Reviewed-by: Martin Peres <martin.peres@mupuf.org>
Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>
https://gitlab.freedesktop.org/xorg/xserver/-/issues/1156
(cherry picked from commit 3b265c59a6)

glamor/glamor.c

@@ -414,6 +414,7 @@ glamor_debug_output_callback(GLenum source,
 
     LogMessageVerb(X_ERROR, 0, "glamor%d: GL error: %*s\n",
                screen->myNum, length, message);
+    xorg_backtrace();
 }
 
 /**