mesa: call FLUSH_VERTICES() before deleting shaders, buffers, query objects

mesa: call FLUSH_VERTICES() before deleting shaders, buffers, query objects

Need to flush rendering (or at least indicate that the rug might be getting
pulled out from underneath us) when a shader, buffer object or query object
is about to be deleted.

Also, this helps to tell the VBO module to unmap its current vertex buffer.