Fill-type functions used to return 0 for success, and -1
on failure. They now return the number of cells written
on success, similarly to ncvisual_render(). Resolves#427.
Resolves#410. notcurses_at_yx() accepted a cell*, but the
gcluster of this cell was always set to 0. The EGC is instead
a heap-allocated copy, returned as the primary return value.
This is due to the absence of an egcpool to bind against.
Existing callers can be converted thus:
* instead of passing cell 'c', pass &(c)->attrword, &(c)->channels
* either initialize 'c' with CELL_TRIVIAL_INITIALIZER, or set its
gcluster field to 0 following the call
I've updated all calls from tests/demos, updated the docs, and
updated the C++ and Python wrappers.
* tetris: use NES gravities
* tetris: use NES grav multiplier of 50ms
* tetris: implement move down #421
* README: mention notcurses-tetris #421
* tetris: use double box for boundary #421
* tetris: extract background.h
* tetris: break up into chunks suitable for book
* tetris: do the rotations
