Auto merge of #3090 - hiikezoe:stop-flicker, r=nical
Auto merge of #3090 - hiikezoe:stop-flicker, r=nical

Don't move pending_properties to current_properties in flush_pending_…

…updates

Basically add_properties is called along with set_properties, but sometimes
add_properties is called without set_properties, once it happens in a single
transaction, the DynamicProperties which was previously set by set_properties
will not be incorporated into the dynamic properties because it was cleared
in the previous transaction, thus a flicker will happen.  E.g. a transform
animation value set by set_properties isn't rendered.

To avoid the flicker this change stops moving pending_properties in
flush_pending_updates and thus only set_properties clears the
penrind_properties.

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