Fix gamma correction for dotstars

[brentru]

When reviewing my code for optimization, I noticed that gamma correction is (incorrectly) only performed on NeoPixels:

  uint32_t rgbcolorGamma =
      strands[strandIdx].neoPixelPtr->gamma32(pixelsWriteMsg->pixels_color);

For DotStar strands, where the default of neoPixelPtr is set to nullptr, this line will not execute at all.

This pull request adds a new getter function, getGammaCorrectedColor() which returns the gamma-correct RGB hex value. getGammaCorrectedColor() works with both NeoPixel and DotStar strands and also provides error handling.