Introduce limited plane rotation capability. We currently support clockwise and counterclockwise rotation of planes. Square and rectangular geometries are both supported, but there must be an even number of columns. The atomic unit of rotation is a 2x1 "square" (this assumes .5 cell aspect ratio). We can only rotate those glyphs which have rotated equivalents, and not even all of those. We currently handle only:
* null glyph
* space
* upper half block
* lower half block
* full block
I've added unit tests as well. This functionality is used by our Tetris example in the book
* introduce doctest over googletest #202
* call dtester in in targets
* doctest conversion #202
* channel.cpp -> doctest #202
* egcpool tests -> doctest #202
* input tests to doctester
* zaxis -> doctest
* drone: always define LANG
* libav to doctest #202
* panelreel tests to doctest #202
* spec that a C++17 compiler is now required for doctest #202
* enmetric tests -> doctest #202
* fade tests -> doctest #202
* notcurses test case -> doctest #202
* last conversion to doctest #202
* finish move to doctest #202
* drone: set up make test