COLRv2

[behdad]

This branch implements various proposed paints for COLRv2.

Currently it has PaintTemplate's:
googlefonts/colr-gradients-spec#371

And PaintGlyphSelf/PaintGlyphDelta:
googlefonts/colr-gradients-spec#370

HarfBuzz counterpart: harfbuzz/harfbuzz#4363

[behdad]

Copying templatization results from googlefonts/colr-gradients-spec#371 (comment):

The savings unfortunately are meager. It saves 2.84% of the COLR table size, which translates to only 0.63% of the total font. This is expected since most glyphs do not templatize in a full emoji repertoire. Here's the tools output:

$ time fonttools colorLib.CORLv1ToCOLRv2 NotoColorEmoji-Regular.ttf 
3719 glyph paints
67532 layer paints
Original COLR table is 1058175 bytes
294 unique templates for 1246 glyphs
Skipped 23 templates as they didn't save space
Building templatized font
3719 glyph paints
58905 layer paints
Reconstructed COLR table is 1028118 bytes
Templatized COLR table is 2.84% smaller.
Saving NotoColorEmoji-Regular.templatized.ttf

real	0m14.381s
user	0m14.126s
sys	0m0.197s

[behdad]

For PaintGlyphSelf / PaintGlyphDelta:

$ fonttools colorLib.COLRv1ToCOLRv2 Nupuram-Color.colrv1.ttf 
841 glyph paints
1682 layer paints
Original COLRv1 table is 27016 bytes
Detecting self/delta glyph paints.
2 unique templates for 841 glyphs
Skipped 2 templates as they didn't save space
Building COLRv2 font
841 glyph paints
4 layer paints
Constructed COLRv2 table is 5192 bytes
COLRv2 table is 80.8% smaller.
Saving Nupuram-Color.colrv1.COLRv2.ttf

cc @simoncozens