expand imageinfo in C api, start on colorspace

After sk_imageinfo was initially created, we have added a colorspace to it,
and that object is a smart-pointer. In response to that, this CL upgrades
imageinfo to an "object" (like paint), so we can manage the ref-counting of
its colorspace, and to allow for future changes/expansion (like paint).

Bug: skia:
Change-Id: I629ff99c0820fdbe83f062d9fb768c15cda68e18
Reviewed-on: https://skia-review.googlesource.com/157156
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
diff --git a/gn/core.gni b/gn/core.gni
index 65d3892..f8d7fdf 100644
--- a/gn/core.gni
+++ b/gn/core.gni
@@ -8,6 +8,7 @@
 _include = get_path_info("../include", "abspath")
 
 skia_core_sources = [
+  "$_src/c/sk_imageinfo.cpp",
   "$_src/c/sk_paint.cpp",
   "$_src/c/sk_surface.cpp",
   "$_src/c/sk_types_priv.h",