fix: match `color.RGBA()` behavior from the standard library (#30)

* Update whisker in flake

The template requires whisker 2.5.1 but in nix we were stuck with 2.3.0

* Make alpha 255 by default

I think this was the intended behavior, we want the colors to be fully
opaque.

* Match the expected behavior of color.RGBA() and update tests

In the standard library color.RGBA() expects returned values to be in
the interval [0, 0xffff]. With this patch we match the behavior of
image.color.RGBA.RGBA().

Tests were updated accordingly and a test was added to make sure we
match the implementation of the standard library.