gainmapmath: write green channel in RGB888 output

[gregbenz]

What changed

• Write pixel.g to the green byte in putRgb888Pixel().
• Add a regression test using distinct red, green, and blue values.

Why

putRgb888Pixel() currently writes pixel.r to both the red and green bytes. Any
RGB888 output where red and green differ is therefore color-corrupted. Writing the
actual green component preserves the requested RGB pixel values.

Testing

• Added GainMapMathTest.PutRgb888PixelWritesGreenChannel; it fails on current main
  and passes with this change.
• Full non-HEIF unit suite: 1,291 tests run; 1,067 passed and 224 existing parameter
  combinations skipped.

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[DichenZhang1]

Thank you for finding and fixing this bug!

[gregbenz]

@DichenZhang1 thank you and the team for all these great enhancements and ongoing support of this critical library for HDR!