st-shadow: Ceil size before comparing with texture

st-shadow: Ceil size before comparing with texture

A fractional resource scale would mean we never use the fast path for
creating the shadow, because we'd cast the int to a float before
comparing, which would never match.

Instead compare the expected texture size with the source texture, to
actually potentially trigger the fast path.

https://bugzilla.gnome.org/show_bug.cgi?id=765011