improve apa102 display fcn

[markoaurelije]

Actually for APA102 LEDs, the start-frame needs to be zeros of 32bit size. For the end-frame n/2 bits needs to be written (n is size of apa102 strip. This improvement reduces the bytes needed to be written resulting in notable increase of speed.

[rm-hull]

Tests fail

[markoaurelije]

Thx, now it should work.
Here is the article this is based on: https://www.pololu.com/product/2556
also your tech-spec/APA102 has similar info about the length of start and end frames.

[rm-hull]

Looks like a worthwhile change .. to paraphrase, rather than creating a buffer of 3n, it just creates a smaller buffer 4 + n + n/2, populates data at offset 4 onwards. It still has the same effect in that it correctly resets the pixels, but takes significantly less time due to the smaller reset data.