evas filters: Prevent redraws when obscure changed

evas filters: Prevent redraws when obscure changed

If the obscured area in a snapshot object changes a lot, do
not try to keep track of it forever. Instead, redraw the filter
over the entire object region, without obscure.

This fixes a performance issue when an opaque window is moved
above a fixed transparent window (the latter has a snapshot with
blur filter).