Add DrawMask: paint one colour through a coverage mask

[tannevaled]

A rasterised glyph is not an image. It carries no colour of its own, only how much of each pixel the outline covers — and text is the one thing every widget draws. A font back-end had to walk that mask calling PutPixel, scaling the coverage by the ink's alpha itself on every pixel, and PutPixel shifts, bounds-tests, clip-tests and blends one at a time.

	ns/op
BenchmarkDrawMask	306,043
BenchmarkMaskPerPixel	700,334

2.3× on 2000 glyph-sized masks, the order a full window of labels issues.

Note what that number is not. There is no row to copy here: a mask has different coverage at every pixel, so unlike FillRect (41×) or DrawImage (6.7×) this cannot skip work, only stop repeating decisions. Two thirds of the gain came from writing the blend out instead of calling blendInto — 561,941 with the call, 306,043 without.

The hand-written blend is the risk, and it bit

Matching blendInto's colour rounding but not its alpha byte — which truncates where the colour bytes round — passed nine hand-picked cases, because every one of them had an opaque ground, where both formulas agree.

TestDrawMaskBlendsExactlyLikePutPixel sweeps all 256 coverages against five ink alphas and six grounds, three of them translucent. Reintroducing the wrong formula makes it fail at coverage 2, byte 3, by exactly one level — so the test is known to catch what it was written for, not merely known to pass.

The other half of the point

A font back-end can now ask for a capability. go-widgets/toolkit asserts *painter.PixelPainter — a concrete type — to render TrueType at all, so any other raster back-end silently falls back to the crude Text primitive. MaskPainter is what lets a recording, damage-tracking or future GPU painter receive real glyph coverage.

100% statement coverage.

🤖 Generated with Claude Code