Fix missing support for drawing the mouse cursor in depth 24 of direct
mode.

Use the general DRAWPIXEL() macro with its bigger case statement
(twice) instead of our big case statement (once). DRAWPIXEL() is more
complicated since it is not missing support for depth 24 or
complications for colors in depth 16 (we currently hard-code black and
white so the complications for colors are not needed). DRAWPIXEL()
also does the bpp calculation in the inner loop. Compilers optimize
DRAWPIXEL() well enough, and the main text drawing method always
depended on this. In direct mode, mouse cursor drawing is now similar
to normal text drawing except it draws in 2 hard-coded colors instead
of 1 variable color.

This also fixes a nested hard-coding of colors. DRAWPIXEL() uses the
palette in all cases, but the direct code didn't use the palette for
its hard-coded black. This only had an effect in depth 8, since
changing the palette is not supported in other depths.