sdl2: Slow the mouse a bit

Fixes Furyhunter/SRB2#1 but probably makes mouse speed too slow on
Linux. Whatever.
sdl2: windows exe rename srb2sdl2.exe
sdl2: fix mouse grabbing in windowed

Should now ignore all motion events and not warp the mouse when the
window is out of focus.
Merge branch 'sdl2' into uncapped