This reduces the time from about 133ms to about 122ms on my test
image on the memento pycamera
a similar change to morph did not produce a performance improvement,
so I didn't include it.
morph9 is a form of morph which performs 9 different convolutions,
like a version of mix where each coefficient is a (2n+1)x(2n+1) matrix.
Most use cases are covered by morph-then-mix, but some advanced operations
may be more efficient to implement via morph9.
This allows operations between channels in an image. It can be used for
the following use cases:
* Conversion to B&W or sepia
* Adding color casts
* Mixing or swapping arbitrary channels
* Inverting or scaling arbitrary channels
bitmapfilter.morph is taken from openmv's imlib.
It is substantially faster than blur/sharpen implemented in ulab,
by up to 10x. It also avoids making many allocations.
