Fix subpixel shift calculation for negative coordinates
Fix subpixel shift calculation for negative coordinates

The subpixel shift calculation was off in case the device coordinates
were negative.  This has been corrected and the calculation been
simplified.

astiob/libass,torque/libass,stps/libass,vault/libass,ShiftMediaProject/libass,asimonov-im/libass,Distrotech/libass,tgoyne/libass,richardpl/libass,blinkseb/libass,xbmc/libass,rhlee/libass,libass/libass,xbmc/libass,pigoz/libass-ct,ShiftMediaProject/libass,astiob/libass,mwgoldsmith/libass,MrSmile/libass,Akemi/libass,grigorig/libass,richardpl/libass,rhlee/libass,stps/libass,Distrotech/libass,rhlee/libass,richardpl/libass,tgoyne/libass,pigoz/libass-ct,Akemi/libass,sanyaade-g2g-repos/libass,mwgoldsmith/libass,blinkseb/libass,pigoz/libass-ct,Akemi/libass,asimonov-im/libass,libass/libass,Distrotech/libass,tgoyne/libass,mwgoldsmith/libass,torque/libass,grigorig/libass,torque/libass,sanyaade-g2g-repos/libass,vault/libass,MrSmile/libass,sanyaade-g2g-repos/libass,stps/libass,blinkseb/libass