Update masks.png.

No more blurred lines! Used a different algorithm to stretch it so that
it's pixelated and not blurred. This cut the file size down to nothing
and made it look better for this purpose.

Trying to keep image updates in their own commit so that, if necessary,
I can go back and delete the commits later.
Tweak color keying.

loadFromFile now accepts another parameter that lets you choose whether
to make a specific color transparent. At this point, I know Keen's
sprites will need to have a transparent background, but I'm hoping the
masks won't need this. This way, I can render the masks as-is but still
render Keen with a transparent background.

Also removed an old helper function that doesn't get used.