Add support for URxvt OSC-11 extension to set background alpha
URxvt has support for specifying alpha in OSC color sequences, either using a
[alphapercent] prefix, e.g.
\E]11;..., or as an rgba sequence (instead of an rgb sequence):
Since foot has support for background alpha, we should implement this. It is used by e.g. pywal to set the alpha (using the
-a <alpha> command line option).
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