Brush strokes record a pen pressure (0..1) per point from egui touch
force while Brush > Pen Pressure is on (default). Each dab's radius goes
from a tenth to all of the brush, and its flow is proportional to the
pressure, in the committed stroke and the live preview alike. A mouse
stroke stores no pressure, so documents and renders are unchanged. The
GPU mask kernel leaves pressure strokes to the CPU. mask.brushStroke takes
`pressure`; bad lists are refused. Also satisfies clippy 1.99's
chunks_exact_to_as_chunks in spots.rs.
Sources: own design.
Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
