mapi: Return NULL function pointers for GL_EXT_debug_marker

mapi: Return NULL function pointers for GL_EXT_debug_marker

Mesa returns a stub function pointer to glAnything for years.
Android framework till API level 30 just uses function pointers
returned from eglGetProcAddress without checking if the underlying
extension is supported. If we return stub pointers for functions
in GL_EXT_debug_marker, Android just uses our stub functions instead
of its own stubs and then fail the dEQP. In the past, the issue
didn't show up because mesa only has limited slots and run out of slots
before Android calls eglGetProcAddress on functions inside
GL_EXT_debug_marker.

Signed-off-by: Lepton Wu <lepton@chromium.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5652>