Implement notcurses_render()

[dankamongmen]

notcurses_render() is a fundamental API point which updates the physical screen to match the virtual screen (this has nothing to do with "alternative screens" as mentioned in #3). It needs be implemented atop the data structure defined in #1 (the cell plane).

For the first pass, blit the entirety of the virtual buffer to the physical screen, using a 1bpc changemask (don't replace character cells that haven't been touched; notcurses_clear() and friends touch all cells). This will validate the basics of our framebuffer.

Next, take advantage of advanced terminal blitting capabilities (clear-to-eol, etc.) if they're available.

Next, ensure we're not emitting more escape sequences than are necessary. Avoiding two passes over the buffer will require keeping some metadata about how far foward a state applies.