transparency

[benjajaja]

There is sixel_dither.set_transparent(i32) to set some palette(?) color to transparent. On its own it does nothing. But if I change the output to start with <esc>P0;1;0q instead of <esc>Pq, then that color indeed becomes transparent, tested on xterm. However this on its own is quite useless.

What I think should happen, is that if e.g. set_transparent(0) has been called, that this keycolor should then be used for any pixel that is fully transparent in the pixels passed to .encode(...) later.