skhdr: Add operator== for several structures

Define SKIA_COLOR_SPACE_PRIMARIES_OPERATOR_EQUAL to avoid compile errors
when integrating to Chromium.

Bug: 395659818
Change-Id: I24f20add4fb9490b1bb954890cafbdb483302c79
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1142376
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Reviewed-by: Christopher Cameron <ccameron@google.com>
Commit-Queue: Christopher Cameron <ccameron@google.com>