Enable bitmaptools on the 'unix' build, 'coverage' variant

[jepler]

This allows testing & debugging of bitmaptools routines like alphablend & dither on host computers. It shouldn't change anything about how displayio/bitmaptools work on microcontrollers.

$ ./micropython-coverage 
MicroPython 7.1.0-beta.0-3-gd55388a17 on 2021-11-12; linux version
Use Ctrl-D to exit, Ctrl-E for paste mode
>>> import displayio, bitmaptools
>>> b = displayio.Bitmap(32, 32, 65536)
>>> bitmaptools.dither(b, b, displayio.Colorspace.RGB565)

[dhalbert]

Q re unneeded default values.

[dhalbert]

Looks good; thanks for experimenting with testing with the Unix port, which will have advantages in the long run. Will wait for a clean build to merge, or you can if you see it first.

[jepler]

Thanks !