rename glyph type locals and other identifiers
type annotations for luaotfload-colors
refactor color expression validation
switch to lpeg based color expression matching
move rgba converter to lpeg; also some caching
improve legibility of node_colorize
fix invalid identifiers
move colorization to ``pre_linebreak_filter``

also introduces a config option: *color_callback* that
controls which callback will be activated. set it to
pre_output_filter to restore the previous behavior.