CMYK destinations: draw into a pair of images, blend subtractively

[timzifer]

Closes #39. For timzifer/cera#209.

API (additions only)

type Paint struct {
	Color  color.RGBA
	Shader Shader
	Second *Paint // what goes into the second image; nil: the same
}

func (c *Canvas) ResetPair(dst, second *image.RGBA, region image.Rectangle)

type LayerShader struct {
	// …
	Subtractive bool // Src and Dst hold ink amounts: blend on complements
	Black       bool // the K image of a pair: K of backdrop (Hue, Sat, Color) or source (Luminosity)
}

Why a pair of RGBA images

A CMYK destination with alpha is two images of the same rectangle, (C, M, Y, A) and (K, K, K, A), premultiplied ink amounts (0 = no ink, so a transparent backdrop is paper). Everything stilus does per pixel treats the four channels alike — over, coverage, clips and masks, gradients, meshes, image sampling — so it is exact on ink amounts, and the existing SWAR and SIMD paths stay as they are. Only blending needs to know about ink (Subtractive), and only the non-separable modes need K apart (Black, PDF 2.0 11.3.5.3). The K image carries K in all three channels, so separable modes give the same K in each.

Canvas

• paint returns a pairBlitter (BlitRun/BlitCoverage forwarded to both) when a second image is set; rasterization, clips, the stroker, shapes and unions are untouched.
• Direct paths: the glyph cache's mask blit writes both images (with the second paint's colour), and its mask-shader path gets a second mask shader; the analytic rows of opaque strokes (seg.solid) step aside for a pair and go through the blitter. Opaque/invisible tests look at both paints.
• Reset drops the second image.

LayerShader

mode() maps the blend mode for the K image (Luminosity → Normal, Hue/Saturation/Color → blendBackdrop, B = cb, an internal value). With Subtractive the generic path complements source and backdrop (α − c), blends, and complements back; Multiply/Screen's integer fast path is for additive layers only. Normal is unchanged.

Tests (cmyk_test.go)

• TestCanvasPair: fills of curves, opaque (analytic) and translucent strokes, a dashed stroke, pixel-aligned and offset rectangles, a shader fill, cached glyphs blitted directly and through the mask shader, under a fractional clip rectangle and a clip path and without clips: the pair gives byte for byte what each image gives drawn alone; with no second paint both get the first.
• TestLayerShaderSubtractive: 400 random premultiplied pixel pairs, all 16 modes, subtractive and (for the non-separable) as the K image, against s·(1−αb) + αs·αb·(1 − B(1 − cb, 1 − cs)) in floating point, within one level.
• go test ./... green. Benchmarks of the single-image paths (glyph cache, border stroke, layer shader, rectangle fill) show only this machine's spread.

Luminosity soft masks of CMYK groups and the conversion of a pair to image.CMYK are left to the caller (cera).