Draw text from an atlas the CPU fills, composed by the GPU

Each size's symbols are drawn on the CPU into one 8-bit atlas, every pixel
the exact area its curves cover, and text is a quad per glyph reading it.
A 1080p page of 11,382 glyphs composes in 1.07 ms at 16 px (it took 525 ms
computing coverage per fragment), and the composed pixels are the CPU
raster's exactly (worst difference 0 for DejaVu's ASCII, 1 for Noto's
11,172 Hangul syllables).

Computing coverage on the GPU lost on both counts: a compute pass filling
the atlas took 5-13x longer than one Cortex-A76 core (35 against 2.7 ms at
16 px), and its faster sorting-network form computed whole rows wrongly
under this Mesa V3D. Coverage now has one implementation, in C, and the
shaders only compose. vega_symbol_raster takes a row stride to draw into
the atlas in place.

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