support scrolling on a per-ncplane basis

[dankamongmen]

Output to an ncplane ought be able to scroll. This will be pretty much necessary, for instance, to support spawning an external process and rendering the output to a plane.

The scrolling model will be that everything moves up one line when either:

• a newline is emitted on the bottom row, at any column, or
• any character is emitted on the bottom row, at the last column, or
• a wide character is emitted on the bottom row, at the penultimate column

When scrolling is enabled on a plane, its rectangular framebuffer is replaced with an array of array of cells, and an integer identifying which one is the topmost line at this moment. Perhaps we ought just always use this structure, and when scrolling is disabled, the integer will always be 0? I see no reason not to, hrmm.

Actually, we can still just use a rectangle for a single allocation, and the integer will just be scaled by the row length. Yessssssss, that's the right way to do it.

Rendering and output will both need to be updated to use this new integer.

[dankamongmen]

This is going to be pretty much necessary for proper implementation of #320, so moving it up to 1.3.0.

[dankamongmen]

I added ncplane_set_scrolling(ncplane*, bool). Updated header, added C++ wrapper, updated README.md, updated notcurses_ncplane.3, added Python wrapper. Now need to implement it.

[dankamongmen]

We now move to the next line when scrolling is enabled. Still need to implement scrolling on the last row, but this ought be enough to implement #320. Making a new bug on the latter problem.