Proactively dispose image filters for SVG filter chains
Because of the spanning of multiple heaps by the resources associated
with FilterEffects [GCd] (SkImageFilter [mallocd]), the garbage
collector only observes a relatively slow growth, while resources tied
by or via the other heap can be substantial.
Since we have fairly good control of the lifetimes here, we can try to
dispose of our references to the resources on the other heap up front,
and prevent growth due to (dead) GCd objects in limbo.
Also rename FilterEffect::ClearResult to DisposeImageFilters to better
match it does nowadays.