Add interesting features to visualize_color_gamut

Add interesting features to visualize_color_gamut

(1) Draw triangles onto a D50 color gamut so they are
    actually in the correct place.
(2) Add options to display canonical sRGB and Adobe RGB
    gamuts.
(3) Label the R, G, and B points of the color gamut.
(4) Decode and reencode the input image without color
    correction, so we can compare to the corrected version.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2017153002

Review-Url: https://codereview.chromium.org/2017153002