Each character image, as defined by the font in the GC, is treated as an
additional mask for a fill operation on the drawable.
The drawable is modified only where the font character has a bit set to 1.
For fonts defined with 2-byte matrix indexing
and used with
each byte is used as a byte2 with a byte1 of zero.
Both functions use these GC components:
function, plane-mask, fill-style, font, subwindow-mode, clip-x-origin,
clip-y-origin, and clip-mask.
They also use these GC mode-dependent components:
foreground, background, tile, stipple, tile-stipple-x-origin,