Improve color space support in SkImage::readPixels()

Correct handling of kGray, k565, k4444 etc. is still a TODO.
SkImage_Generator and SkImage_Gpu are still TODOs.

BUG=skia:6021

Change-Id: Ib53d97d3a866b2b4934fd85c10100855743a8fab
Reviewed-on: https://skia-review.googlesource.com/6396
Commit-Queue: Matt Sarett <msarett@google.com>
Reviewed-by: Mike Reed <reed@google.com>
diff --git a/gn/gm.gni b/gn/gm.gni
index 8beb3e9..327416c 100644
--- a/gn/gm.gni
+++ b/gn/gm.gni
@@ -228,6 +228,7 @@
   "$_gm/poly2poly.cpp",
   "$_gm/polygons.cpp",
   "$_gm/quadpaths.cpp",
+  "$_gm/readpixels.cpp",
   "$_gm/recordopts.cpp",
   "$_gm/rectangletexture.cpp",
   "$_gm/rects.cpp",