Most emoji fonts are massive glyphs (128x128, 256x256, etc.). This means
the texture we need to store emoji is also massive. For a 128x128 emoji
font (both Apple and Noto), we can only store 12 emoji before resizing
prior to this commit.
This commit now threads through a max height through to the font face
and resizes the bitmap in memory before putting it in the atlas. This
results in significant savings. The max height is the cell height. We
allow the glyphs to be as wide as necessary due to double (and more)
wide glyphs.
For the unicode emoji test file, the atlas size before and after:
Before: 262 MB
After: 16 MB
This is more oriented around glyph indexes and also introduces an
important concept in the FontIndex which can be cached ahead of time so
that we can eventually break down text into runs for text shaping.
