sixel: implement private mode 1070 (private color registers)
Private mode 1070 controls whether each sixel has its own set of color registers, or if the color palette is re-used between colors.
Ps = 1 0 7 0 ⇒ use private color registers for each graphic.
Foot currently used a shared color palette. XTerm default is set, i.e. private color registers.
In practice, foot's implementation is not likely to cause any problems. Programs that do not use shared color registers typically emit a custom color palette before each sixel.
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