- fix misleading hint about minimising on focus loss
Mac: Hide cursor in relative mode.

This hides the cursor when you SDL_SetRelativeMouseMode, as intended.

Fixes http://bugzilla.libsdl.org/show_bug.cgi?id=1860
Mac: Fix incorrect relative jump on focus / start.

We should no longer send huge jumps on relative mode focus changes if
the cursor was moved around, or on initial start.

Fixes http://bugzilla.libsdl.org/show_bug.cgi?id=1836