Add ability to change glyph but retain coloring/styling

[dankamongmen]

I suppose this is the glyph analogue to ncplane_stain() (see #347 ). Add a means by which glyphs can be changed, but colors can be retained. Note that this is not going to work exactly as expected on top of ncvisuals, where the glyph is part of the "color". We're already cool with that in the case of transparent text atop an ncvisual, though.

This is basically equivalent (modulo plane movement etc.) to making a transparent plane atop what we're changing, and writing transparent text.

[dankamongmen]

So by "change glyph", the primary way that's done is with e.g. ncplane_putc(). Are we suggesting there ought be a way to invoke this family of functions such that the channels are unaffected? That starts a pretty nasty combinatorial explosion--we already have _yx() and _aligned() calling down to a common form for each of these.

I'm thinking maybe #361 is enough to throw together a generic solution for this. It can maybe be made available as an inline, but I don't even like that clogging of the namespace for this rare need.

Yeah, I'll add a notcurses_putc_stainable() to do a single EGC, since there is no notcurses_putc_aligned(). Beyond that, whip up a transparent ncplane, write to it, and merge it down. It's slower, but this just doesn't seem like something that will happen often.