Fix inset box shadows with blur
Fix inset box shadows with blur

In commit d57057470cb2 ("Rework box shadows to support border radii
properly ..."), the conditional of the box shadow fragment shader that
handles inset shadows was lost. Restore it.

servo/webrender,servo/webrender,servo/webrender,servo/webrender,alanamramjit/webrender,alanamramjit/webrender,servo/webrender,servo/webrender