ncplane_hline() does not work with palindex channels

[tstack]

The ncplane_hline() code assumes the channels contain RGB values:

https://github.com/dankamongmen/notcurses/blob/3a3c75e65ac8342f10a0e16eb63ab642aaf52ef4/src/lib/notcurses.c#L2166-L2175

So, you get garbage when trying to draw a line with palette indexed colors.

Please include the following data:

• notcurses version (available from notcurses-demo i)

v3.0.11