In the category of "how did this ever work"...

TL;DR: in general, Halide Buffers have the opposite axis ordering from Python/NumPy buffers; in Halide, the most-frequently-varying dimension comes first, while in Python, it comes last. This isn't surprising, though, since Halide's indexing scheme is effectively column-major while NumPy's is row-major.

Anyway: what we *should* have done was to reverse the order of dimensions when converting to/from Halide Buffers vs Python buffers; instead, we kept the same order, then jumped thru hoops to rearrange buffers to fit this setup. This PR does the appropriate axis reordering, fixing the apps and tests as needed.

It also adds some helper code for image reading and writing; by default, we use `imageio` for this, but imageio ~always wants RGB/RGBA images to be interleaved (vs the planar that Halide prefers). So, I added the `halide.imageio` package, that has wrapper functions to quietly convert to/from planar as needed.

Needless to say, this change is likely to break existing code that is using 3d buffers in Halide, but I think it's the right long-term thing to do.

Opinions greatly welcomed here.

* add 'reverse_axes' options to Buffer conversions