exo: Prevent shaped windows from dimming the shelf.
exo: Prevent shaped windows from dimming the shelf.

This prevents shaped windows from dimming the shelf unless the shape
occupy the whole area of the widget.

BUG=549781
TEST=exo_unittests

Review URL: https://codereview.chromium.org/1877723002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#386310}

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