T25c Generative: canvas

InpaintCanvas lays a mask over a picture: brush strokes in shares of
the picture, rastered into a 320-column grid where a later stroke wins
and an erasing one clears, painted as merged rectangles snapped to
device pixels. A ring follows the pointer at the brush's size; [ and ]
size the brush and X swaps it when focused. MaskBrush sets paint or
erase and the size, with Undo and Clear waiting for a stroke.

Pointer strokes move to forms::pen, which SignaturePad and the canvas
share.

tasks/ch21-30.md

@@ -227,8 +227,8 @@
 
 ## T25c Generative · Canvas
 
-- [ ] InpaintCanvas
-- [ ] MaskBrush
+- [x] InpaintCanvas
+- [x] MaskBrush
 
 ## T25d Generative · Sound and motion