~rabbits/uxn

remove chr2img (see https://git.sr.ht/~rabbits/chr-format instead)
chr2img: shorten writebmp, explain bitmasks
format chr2img.c, weirdly
chr2img: port to Unix-like operating systems
Further fixes to keyboard handling
Fix keyboard problems introduced in improvements

Handling of a lot of ordinary keys was broken, e.g. number row and
non-alphanumeric keys, especially with Shift.

I think what's broken has been fixed, but have reverted the improvement
that Controller/vector is only called once per keypress. Many keys cause
Controller/vector to be called twice, but only once with Controller/key
nonzero.
Improved syntax highlight
Various keyboard handling improvements

The Ctrl, Alt and Shift bits in Controller/button activate with either
left or right modifier keys, and either can be used to take screenshots
/ zoom / show debugging info rather than left modifiers only.

Delete now sets Controller/key to 0x7f rather than being ignored (by
setting Controller/key to 0x00).

Ctrl + letter now sets Controller/key to 0x01-0x1e rather than being
ignored: for example, Ctrl+S will set Controller/key to 0x13.

Controller/vector is no longer called twice per key press (the extra
call with Controller/key set to 0x00 is removed).
Added return pointer position to debugger
Updated README with screenshot hotkey change
Reworked H key handling into switch statement
Fixed critical issue where a label/macro with more than 255 refs would vanish
Give ids to labels and macros
Improved uxnasm error report
mkfile: fix SDL2 cflags
Changed capture button
Removed lock/unlock for screenshot
Fixed order of unlock/save
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