Add general ncplane_line_yx()

[dankamongmen]

We have ncplane_hline() and ncplane_vline() and they're both kinda dumb. Why don't we just have an ncplane_line_yx() and figure things out for ourselves? These limiting cases would be the easiest to solve for fully general line-drawing (where, of course, the biggest question is: what glyphs do we use at what angles?).

We can leave ncplane_hline() and friends as wrappers, of course.

[dankamongmen]

// from current cursor position
ncplane_line_yx(struct ncplane* n, int y, int x, const cell* c, uint64_t destchan);

that ought handle it, methinks. the idea of using distinct glyphs at different points is crazy. let the user pick one and be done with it. we draw from the current cursor position, and the first cell matches c exactly. further cells continue to use the EGC and attributes from c, but lerp towards destchan. if destchan == c->channels, the line is a single color.

yeah, that's good.

[dankamongmen]

I don't think this will be genuinely useful. Tabling it for now.