Load images- untested.
Can paint 100% transparent pixels with 'g', draws checkers underneath.  TBD make sure alpha convention matches what gimp will show when it loads a saved image.

Can load images with command line (untested, plus how to pass args without exporting app first?), or with 'o' selection box (how to provide default path, or use last used path?).

Clearer display of color palette.

Can move diagonally with yunm (TBD should be yubn).
properly saving alpha channel, and just edited image not entire screen.
corrected diagonal keys
Add shift ability, where image shifts vertically or horizontally and repeats over edges, it ought to be shifted up so that position under cursor is drawn on in drag_mode.

Drag mode ought to be a left hand key (or more central right or left key) since shift keys are right handed, user will probably toggle drag and colors very rapidly.

Next, animation.
drawing little preview (need to draw it uninterpolated), also making checkerboard background an image instead of redrawing it every frame.

optional draw grid, don't interpolate the thumbnail
Make function take an PImage argument, in advance of implementing animation