SkQP: fix model colorspace (16-bit gold images)

Problem: `make_gmkb.go `was ignoring the ignoring the embedded ICC
profile in the images it was getting from Gold.

Replace make_gmkb.go with two small programs: `goldgetter.py` and
`make_skqp_model.cpp`.

`make_skqp_model` uses Skia to create the model from a bunch of images.

`goldgetter` wraps `make_skqp_model` and handles:
  - json parsing
  - downloading images from gold
  - multiprocessing

CQ_INCLUDE_TRYBOTS=skia.primary:Build-Debian9-Clang-x86-devrel-Android_SKQP,Test-Debian9-Clang-NUC7i5BNK-CPU-Emulator-x86-devrel-All-Android_SKQP

Change-Id: I7add1a1dfd83bbd0ab07ab126d4183c36325263c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/209101
Commit-Queue: Hal Canary <halcanary@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
diff --git a/BUILD.gn b/BUILD.gn
index d5f43f9..51d3510 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -1812,6 +1812,15 @@
     }
   }
 
+  test_app("make_skqp_model") {
+    sources = [
+      "tools/skqp/make_skqp_model.cpp",
+    ]
+    deps = [
+      ":skia",
+    ]
+  }
+
   if (target_cpu != "wasm") {
     import("gn/samples.gni")
     test_lib("samples") {