ncplane_hline() does not work with palindex channels

[tstack]

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

notcurses/src/lib/notcurses.c

Lines 2166 to 2175 in 3a3c75e

	unsigned ur, ug, ub;
	int r1, g1, b1, r2, g2, b2;
	int br1, bg1, bb1, br2, bg2, bb2;
	ncchannels_fg_rgb8(c1, &ur, &ug, &ub);
	r1 = ur; g1 = ug; b1 = ub;
	ncchannels_fg_rgb8(c2, &ur, &ug, &ub);
	r2 = ur; g2 = ug; b2 = ub;
	ncchannels_bg_rgb8(c1, &ur, &ug, &ub);
	br1 = ur; bg1 = ug; bb1 = ub;
	ncchannels_bg_rgb8(c2, &ur, &ug, &ub);

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