Better fill algorithm for BOX's fill mode

[dansanderson]

Is your feature request related to a problem? Please describe.
The BOX command has a "fill" mode that is currently implemented as a series of lines. This leaves gaps in some cases.

Describe the solution you'd like
Use a more thorough polygon fill algorithm.

Describe alternatives you've considered
We've speculated about replacing or augmenting BASIC's graphics subsystem to use a different graphics mode where DMA could be used for much faster line drawing. This could offer more ways to draw filled boxes.

The BASIC bitmap graphics subsystem is not our highest priority, so it's possible this won't see improvement any time soon.

Additional context
Originally reported here: https://www.forum64.de/index.php?thread/128028-bug-report-box-command-with-solid-option/

See especially Mike's reply for an example fill algorithm.