Xmir -rootless: glxinfo reports "Failed to open BO for returned DRI2 buffer (100x100, dri2 fake front buffer, named -1)."

[Daniel van Vugt]

When Xmir is running -rootless, glxinfo reports errors:

OpenGL core profile extensions:
    GL_3DFX_texture_compression_FXT1, GL_AMD_conservative_depth,
    GL_AMD_draw_buffers_blend, GL_AMD_performance_monitor,
    GL_AMD_seamless_cubemap_per_texture, GL_AMD_shader_trinary_minmax,
    GL_AMD_vertex_shader_layer, GL_AMD_vertex_shader_viewport_index,
    GL_ANGLE_texture_compression_dxt3, GL_ANGLE_texture_compression_dxt5,
    GL_APPLE_object_purgeable, GL_ARB_ES2_compatibility,
    GL_ARB_ES3_1_compatibility, GL_ARB_ES3_compatibility,
:Failed to open BO for returned DRI2 buffer (100x100, dri2 fake front buffer, named -1).
This is likely a bug in the X Server that will lead to a crash soon.
Failed to open BO for returned DRI2 buffer (100x100, dri2 fake front buffer, named -1).
This is likely a bug in the X Server that will lead to a crash soon.
    GL_ARB_arrays_of_arrays, GL_ARB_base_instance, GL_ARB_blend_func_extended,
    GL_ARB_buffer_storage, GL_ARB_clear_buffer_object, GL_ARB_clear_texture,
    GL_ARB_clip_control, GL_ARB_compressed_texture_pixel_storage,
...