reset the state to default

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

this function is designed to support animated gifs, although stb_image doesn't support it

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

after we see SOS