Add 32 bpp truecolor support.

This corresponds to the 32 bpp patch from my old Windows port.
It is integrated in a way which allows use of either 8 bpp or
32 bpp depending on the --truecolor command line flag.

The 32 bpp mode uses a large 32 bits * 65536 (256 KB) lookup table
for converting from the two dimensional internal colour space to
RGB. The table is constructed in setupPalette() in ui.cc and
passed via screen->setPalette(). It uses the most common
arrangement, ignoring SDL_PixelFormat.
Change ordinary fading so it fades all the way down to 0.

Ordinary fading did not fade all the way down to 0. This was not
a problem with 8 bpp output because it uses 4 bits of each of the
two calculated components. That ignored the bits that stayed set.
32 bpp output uses all 8 bits and so fading to 0 is needed.