bugfix: VGA chain4 CRTC rendering should over-render if panning. the
video start given can in fact have bits 0-3 nonzero, so it needs to mask
them off and put the masked-off bits into the panning offset. this fixes
the "flashing blinds" effect that happens whenever a DOS game or demo
does horizontal VGA panning (hpel+offset). Also fixes the "ragged right
edge of the screen" effect in Surprise! productions COPPER.EXE.