When using a snapshot object we have access to exactly all
the pixels below it inside the snapshot surface. So, in order
to produce a nice blur, it is necessary to expand this snapshot
and then clip it otherwise the edges will look a bit ugly.
Unfortunately, there are still places where blurs didn't look
so good, as objects below an opaque region would never get
rendered into the snapshot object. So the edges, inside a
snapshot object, around an opaque region would have blur
artifacts.
This fixes that by shrinking the cutout regions by the radius
of the filter. Eg for blur this is the blur radius.
The test case in elm_test can exhibit this fix very clearly:
a red glow would be visible around the opaque rectangle, but with
these changes we instead see the blurry edges of the objects
below the rectangle.
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