Added:
* Plane: gradient (`ncplane_gradient`)
* Plane: gradient_sized (`ncplane_gradient_sized`)
* NotCurses: drop_planes (`ncplane_drop_planes`)
* NcReel: constructor which takes `Plane&`
* Visual: constructors which take `Plane const*`, `Plane&` and `Plane const&`)
* ncpp_build: a nonsensical "demo" which exists purely to test whether
the C++ builds and does absolutely nothing interesting.
Broke:
* All exceptions throw temporary objects instead of allocated
instances. Less typing in `catch` :P (and more conventional)
