Mouse clicks change opacity of text / other stuff

1) By left-clicking or right-clicking, you can change the transparency
of the background. Very useful for some situations.

2) Fixes the cheat to beat the level.

3) Show when a given sprite will be hittable or not. Not that some
sprites, like the Key, interact with Mario all the frames, but its
hitbox still blinks in the script. The demands a case-by-case
configuration...