Workaround for SDL relative mouse movement switching bug

When switching between absolute and relative mouse modes, the cursor
does the right thing, but the MOUSEMOVE events have bad figures
for .relx and .rely

I spent hours trying to get this working correctly, pulling apart
the SDL source code, trying warps, tweaks to values and mouse warps

Eventually, I settled on this hack. It causes the first mouse move
event to be ignored using a nasty little flag.