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/forms/mod.rs

@@ -95,6 +95,7 @@ pub use rating::Rating;
 pub use search::SearchInput;
 pub use select::Select;
 pub(crate) use select::{Listing, field_button, field_text, listing};
+pub(crate) use signature::{Drawing, pen};
 pub use signature::{SignaturePad, Stroke};
 pub use slider::{RangeSlider, Slider};
 pub use stepper::Stepper;