Make backgrounds foreacg digit transparent while rendering, but use backgroundColor for the entire display's background. This gives more freedom when skewing digits, or when adjusting letter-spacing.

This change shows some pixelation artifacts. However, I develop over NX, so it is likely an artifact of NX compression. I would appreciate it if someone without NX could confirm it look OK. It would save me lugging out a monitor, keyboard, & mouse for my server. :-)