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Move the zcr_alpha_compositing interface into its own files. This
change only moves code around, no functional changes.

Bug: 896710
Test: Build and run exo_unittests
Change-Id: I6b576d9e686c8634dfa2a30c3718f9068628a9ea
Reviewed-on: https://chromium-review.googlesource.com/c/1481733
Commit-Queue: Daniel Nicoara <dnicoara@chromium.org>
Reviewed-by: Daniele Castagna <dcastagna@chromium.org>
Cr-Commit-Position: refs/heads/master@{#635519}