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/mod.rs

@@ -1,5 +1,7 @@
+mod brush;
 mod compare;
 mod grid;
+mod mask;
 mod prompt;
 mod queue;
 mod ratio;
@@ -9,8 +11,10 @@ mod style;
 mod tests;
 mod variation;
 
+pub use brush::MaskBrush;
 pub use compare::{ABCompareView, Verdict};
 pub use grid::{GenerationGrid, Outcome};
+pub use mask::{InpaintCanvas, MaskStroke};
 pub use prompt::PromptEnhancer;
 pub use queue::{GenerationQueue, Job, JobState};
 pub use ratio::AspectRatioPicker;