Not drawing white pixels

It would be nice to have a feature to not draw white pixels, like you can do with some picture formats such as gif. To draw “White”, we would draw a slightly off-white color. That could save a lot of RAM for masking pictures, instead of adding a whole 8bits per pixel of alpha just to do a yes or no mask.

It would be even more useful when combined with color-index mode, because then we could get an mask + colors in only 8 bits.

Consider the R5_G5_B5_A1 format.
Only one bit alpha.