Font: tweaking the debug rects

reserve red for errors, use green for the available area instead.
also enable the debug flag by default for now.
this is in a state of development where this is needed more often than not and it helps me see if changes are *really* good before commiting them
SDLVideo: fix BlitSurfaceClipped

if the destination gets clipped, then we need to clip the source to compensate
fixes top line of text getting FUBARd when scrolling
Font: fix dropcap "disappearing" when scrolled

the optimization to not print when the line is offscreen will cull the cap because technically the cap lineheight is only ~13px