Bound shape recording, mip and glyph cache accounting

[timzifer]

Fixes #30, #31, #32.

#30 Shape geometry budget

• New Shape.MaxBytes (default DefaultShapeBytes, enough for DefaultMaxEdges edges) bounds each form's edges, strips and row bins. A stroke makes up to two forms, so at most 2× MaxBytes.
• Budget is checked before each append; recording stops when full, before bins are built. addPath and the stroker (new Stroker.halt, checked between subpaths) stop early.
• FillShape (fill and stroke) and FillUnion report ErrEdgeBudget for a truncated form.
• The bound belongs to the shape, not the canvas, so canvases with different clips and MaxEdges get the same truncated form on concurrent first use.
• Known gap: the stroker's own per-subpath scratch within one huge subpath stays unbounded, as with Canvas.Stroke.

#31 Texture.MipBytes

• Sums the actual rounded level sizes instead of W*H/3, saturating on overflow.
• Also counts wrap levels that are distinct from ordinary levels (aliased ones once). Hence 4096×1 now reports 32752 (levels + wrap levels), 256×256 stays 87380.
• cera picks this up after bumping its stilus dependency.

#32 GlyphCache.MaxBytes

• Entries count cap(Pix), so reused larger buffers are accounted correctly.
• A mask larger than MaxBytes is drawn but not cached; it is recycled afterwards.
• MaxBytes doc states what is not counted (scratch, map).

Tests

New budget_test.go: MipBytes vs. actual levels (RGBA/gray/alpha/indexed, thin/odd/NPOT), oversized glyph entry, large-to-small buffer reuse, budget reduction, shape fill/stroke/union truncation, concurrent first use. TestGlyphCacheEviction now enforces the budget strictly. go test ./... passes, new tests also under -race.