API click: on mouse down, move the cursor to the clicked location, and reset the selection

API cut: delete selection

discard the oldest entry in the redo list--it's bad if this ever happens, but because undo & redo have to store the actual characters in different cas

API drag: on mouse drag, move the cursor and selection endpoint to the clicked location

API initialize

API key: process a keyboard input

API paste: replace existing selection with passed-in text

build a table that decodes both magnitude and value of small ACs in one go.

decode image to YCbCr format

@OPTIMIZE: store non-zigzagged during the decode passes, and only de-zigzag when dequantizing

returns 1 if "a*b + add" has no negative terms/factors and doesn't overflow

after we see SOS