CTX_GSTATE_PROTECT for sanity checking on save/restore balance (the sanity
checking we already get, with ctx_gstate_protect() and ctx_gstate_unprotect()
inserted around "user" code, balance will also be restored.
CTX_FORMATTER - for recording vector "screenshots" and transmitting display
contents over TCP.
Pixel encodings GRAY1, GRAY2, GRAY4 - for efficient large lowbith grayscale
textures, we've got lots of memory, but we can stretch it much further by
using 1-4bit per pixel rather than 24/32 for some high-contrast use cases.
Pixel encoding GRAYA8 - to handle grayscale PNGs with alpha.
