Rather than inspecting CMAKE_BUILD_TYPE manually and setting
DEBUG_OPTIONS based off the results, set CMAKE_{C/CXX}_FLAGS_DEBUG,
and allow that to be picked up by CMake #565. Remove repeated -W
flags that were all over the place, making them global compiler
flags. Remove repeated FORTIFY=2 definition that was all over the
place, making it a global compiler definition. Remove all uses of
${DEBUG_FLAGS}. Also, since CMake doesn't define a CMAKE_BUILD_TYPE
by default, instead just setting neither debugging nor optimization
flags, in what seems a pretty fucking boneheaded move, set it by
default to RelWithDebInfo. ugh. Explicitly disuse unused PoC variables.
Unify ffmpeg/oiio/null implementations, where possible. This effectively
required placing all three in the same file, which meant they're all now
C++. Update FFmpeg implemenation to be C++-usable. Implement
ncvisual_rotate_cw() and ncvisual_rotate_ccw() #515. Move most of tetris
over to Visual from Plane #558. Add bgra_to_rgba(), necessary for
creating ncvisual from BGRA memory. Implement ncvisual_from_rgba()
and ncvisual_from_bgra() #557. Add unit tests on ncvisual rotation.
Added:
* class FDPlane (`ncfdplane*`)
* class Subproc (`ncsubproc*`)
* NotCurses: get_inputready_fd (`notcurses_inputready_fd`)
* Plane: qrcode (`ncplane_qrcode`)
* class PlotBase: templated base class for Plot variations
* class PlotU: `uint64_t` instantiation of PlotBase (aliased to previous
`Plot` class for source compatibility), `ncuplot*`
* class PlotD: `double` instantiation of PlotBase, `ncdplot*`
