The brush commits via a GPU-canvas readback and relies on the action's first
execute() to SET raw_pixels from that readback — at that point raw_pixels is empty
(new keyframe) or the pre-stroke state, never something a dirty-rect diff can stamp
onto. My initial diff-only execute skipped (to avoid corruption), so the stroke
disappeared.
Fix: the action keeps the full post-edit buffer ONLY for the first execute (the
commit), assigning raw_pixels outright exactly like the old code; it's taken/dropped
immediately, so the action sitting in the undo stack still retains just the small
diff. Redo replays via the diff onto the now-resident base.
Also harden the diff itself for the blank-base case: `before_blank` lets apply_after
build from a transparent buffer (redo of a first stroke after undo-to-blank) and
apply_before restore to empty; the resident-base skip is kept only for non-blank
bases (faulted in before undo/redo). Tests cover commit/redo from empty.
31e23b0fc7