gsk: Fix clipping error when drawing shadows

gsk: Fix clipping error when drawing shadows

When shadows were offset - in particular when offset so the original
source was out of bounds of the result - the drawing code would create a
pattern for it that didn't include enough of it to compose a shadow.

Fix that by not creating those patterns anymore, but instead drawing the
source (potentially multiple times) at the required offsets.

While that does more drawing, it simplifies the shadow node draw code,
and that's the primary goal of the Cairo rendering.

Test included.