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.
* fedora: dep on OpenImageIO, and use it
* fedora: dep on libqrcodegen-devel
* fedora: BuildRequires OpenEXR-devel
* tight check on USE_MULTIMEDIA
* CMake: enable notcurses-view for ffmpeg OR oiio
* notcurses-view: don't reach into libav
* oiio: ncvisual_render() #453
* oiio: need our own properly-offset ncvisual_plane()
* `visual` poc: accept optional command line argument
* oiio: work for 3-channel images #453
* oiio: destroy ncvisual's plane if we own it #453
* notcurses_visual.3: s/FFmpeg/multimedia/g
* ncplane_at_* and ncplane_at_cursor_*
We had notcurses_at_yx() expanding into three distinct parts of
the cell structure, and ncplane_at_yx() / ncplane_at_cursor()
writing directly to a cell. It was annoying to remember which
was which. The latter two now have a signature matching
notcurses_at_yx(), while the old functionality has been moved
to ncplane_at_yx_cell() and ncplane_at_cursor_yx(). #476
nick black