Auto merge of #3481 - gw3583:binding-deps, r=staktrace
Auto merge of #3481 - gw3583:binding-deps, r=staktrace

Fix caching display lists with opacity bindings that are values.

Most of the time when property bindings are used, they are bound
to a specific key that can be animation. However, they can also
be set to a fixed value.

It was previously possible for a property binding with a fixed
value to not be included in a cached picture dependency. This
meant a stale tile could be used in cases where a new display
list arrives with the same property binding IDs but different
values, where everything else is the same.

This patch includes fixed value property bindings in the tile
descriptor's opacity binding dependencies.

This is a fix for https://bugzilla.mozilla.org/show_bug.cgi?id=1518050

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