Comment # 12 on bug 75061 from Marek Olšák

All writes to valid_buffer_range are protected by a mutex. Only the reads are
not.

I've got no idea what to do with invalidate_buffer. If we added mutexes
everywhere, it would slow down the driver.

I think that calling BufferData in one thread and using the buffer for
rendering in some other thread is a race condition in the application and
should be fixed in the app.