In commit d570574 ("Rework box shadows to support border radii
properly ..."), the conditional of the box shadow fragment shader that
handles inset shadows was lost. Restore it.

The box shadow shader will produce the wrong result in that case, and
there is actually no edges to fill.

A positive spread value makes an inset shadow smaller, so take the clip
mode into account when computing the box shadow size.

Fixes #183

v2: Don't introduce extra variable.