In many important use cases (e.g. video playback on youtube) the
display list doesn't change very often. Typically, the only
thing changing between frames are the natively decoded video
texture surfaces.

In these cases, we can save a significant amount of CPU usage
and battery power by not building a frame at all.

Previously, any call to set/update dynamic properties would
trigger a frame rebuild. However, it's often the case that
Gecko sends dynamic properties updates that are either empty
or exactly the same as the previous frame.

With this change, WR will compare the scene properties and
only trigger a frame rebuild if they have actually changed.

This is a partial step towards fixing #2917 - a couple of
other changes are also needed to completely avoid a frame
rebuiild in this use case.