Make forced alpha an optional flag for any image or paint layer

[sbrin]

Currently we always treat white or transparent pixels as empty ones.

It may be useful to be able to add non-transparent but invisible pixels

As a source we expect people to use image drawn in black with white or transparent background

By default process them as if white==transparent (empty), black==filled

With a new layer flag we must process them as white==white (filled), black==filled

For FlipperZero there should be no flags. Always make white==transparent

To be discussed how to name such flag