We need to flush all pending mouse move events past a mouse warp to avoid

We need to flush all pending mouse move events past a mouse warp to avoid
a race condition ending in warping twice and having huge increments for the
second warp.
I am not aware of such a flush function in glut. So we emulate that by
ignoring mouse move events between a warp mouse and the next frame.
That should make glut behavour aequivalent to sdl behavour.