…e texture

Currently at default settings this will hold at least 4096 glyphs, which should be more than enough for all but the most extreme cases.
This is preparation for mixed-font text blocks, and for GPU-rendered SDFs.

…amCacheKey

This fixes key collisions when using the same derivation on different base materials.

…als as of Three r130

Three r130 introduced a "feature" which forces DoubleSide transparent materials to be
rendered in two draw calls instead of one; there's no advantage to this for flat planes,
and it can be a significant performance hit. This works around that by making the geometry
two planes back-to-back, always rendered as FrontSide, and emulating the intended sidedness
via drawRange.