Bug 1397083 - Don't use CopySurface when drawing theme with nsNativeThemeGTK. r=lsalzman

CopySurface doesn't respect any existing clips that are applied to the draw
target. It doesn't look like nsNativeThemeGTK uses CopySurface normally, but
when OMTP is enabled the draw target is a DrawTargetCapture which can't
LockBits() which causes us to use CopySurface. This can cause a reftest failure.

MozReview-Commit-ID: 1mYMhQB0r3M

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extra : rebase_source : d2683dee37ce32049d8daeee786cbb06ba624438
extra : amend_source : 59c48b66f733108636d52047a55889ee0c4e2746