tic80: brighten, fix fractional scaling issues.

Brighten icon's features to improve contrast some more, and match
other icons. Softening it that way wasn't looking sharp in lists.
Using just a 2-tone scheme is probably enough.

Fun fact: pixel and sub-pixel shapes, such as drop shadows from one
layer down to the next, sometimes don't quite align in some renderings
at 150% scaling. This can cause bleed-through of the underlying colour
in odd places. Better to build shapes as complete logical layers, with
bigger overlaps from one layer down to the next, and then copy each
layer for its shadow. Papirus basics, really.

Highlights will be affected by the same glitch, but arguably it matters
less for those.

Author: achadwick