gallium: expose a debug message callback settable by context owner

This will allow gallium drivers to send messages to KHR_debug endpoints

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>

src/gallium/docs/source/context.rst

@@ -84,6 +84,9 @@ objects. They all follow simple, one-method binding calls, e.g.
     levels. This corresponds to GL's ``PATCH_DEFAULT_OUTER_LEVEL``.
   * ``default_inner_level`` is the default value for the inner tessellation
     levels. This corresponds to GL's ``PATCH_DEFAULT_INNER_LEVEL``.
+* ``set_debug_callback`` sets the callback to be used for reporting
+  various debug messages, eventually reported via KHR_debug and
+  similar mechanisms.
 
 
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