New GM that exercises gamma-correctness in various parts of the pipeline.

New GM that exercises gamma-correctness in various parts of the pipeline.

Attempts to render 50% grey through many different techniques. Only the first one (black/white dither) is guaranteed to be correct. This serves as ground-truth for all the others (independent of whatever your display may or may not be doing when viewing the output of the GM).

Current tests:
- Texture bilerp
- Texture scale (ie mipmaps)
- Simple paint color
- Gradient
- A handful of Xfer modes
- 50% grey bitmaps, both linear and sRGB
- Bitmaps wrapped in shaders, which hits a different raster code path

TODO:
- Test coverage (rather than alpha)?
- Text, Blur, etc...?

Re-landing this without the blitter selection change, which may have caused a regression. Also updated the text color so it's legible against both black (SampleApp) and white (gold).

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1740213003

Review URL: https://codereview.chromium.org/1740213003