It got dropped accidentally in commit
6e5fe71076fe38e5236f68d4c05dcc5bcbc4dbd7 since the function the
fix happened in was renamed.

I accidentally dropped the FIXME when I wrote my comments
in the last commit.

It took me a while to understand how the gradient code was
preventing banding, so this commit just adds some comments
explaing how it works, and moves some constants to preprocessor
defines for clarity

It's sometimes useful to specify an opacity to fill with for the
passed in color, too, independent of its alpha.

The current function for filling the frame buffer with
a block color takes 4 doubles.  In some cases, it's
easier to give the hex value directly.