Implements "High Bit Depth Gamma Correction Algorithm for APA102/Dotstar LEDs" for the Adafruit Dotstar library.

[zackees]

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This is a code-style review. Right now the compilation tests pass. However, the way I've implemented this into the main object may not be exactly what the Adafruit team has in mind. So this PR at this time is to do a check-in and see if the way I've implemented is ok, or whether I should change it. If the style is ok, then I'll do final testing with a physical unit. Note that I've already done physical device testing with this change for the FastLED library PR and so i expect the testing to be straightforward and work on the first try.

• Describe the scope of your change--i.e. what the change does and what parts
  of the code were modified.

  This implements the "High Bit Depth Gamma Correction Algorithm for APA102/Dotstar LEDs"

• Describe any known limitations with your change.

This is portable c code implementation. However, I do add a gamma16 function which is 256*sizeof(uint16_t) units in prog memory.

• Please run any tests or examples that can exercise your modified code.

I've added a new test .ino file. Please see onboard_hd.ino