Enable blending on render targets.
Enable blending on render targets.

It was disabled as a fix for #166, but that caused regressions on pages
that depend on blending. The issues related to the blur on box shadows
discussed in that issue seem to be resolved now, so the clear-color
solution works fine.

Addresses browserhtml/browserhtml#1068.

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