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.

src/generative/tests/mod.rs

@@ -2,6 +2,7 @@ use gpui::{KeyBinding, KeyUpEvent, Keystroke, TestAppContext, VisualTestContext}
 
 use crate::{forms, primitives::FocusNext, theme::Theme};
 
+mod canvas;
 mod results;
 mod settings;