The existing behavior was surprising for users who draw opaque objects
into a semitransparent views, and this was especially evident with
the labels in `TextureLinearInterpolationTest`.
We now disable blending for MASKED, which is what our existing materials
documentation already says. We also now set the fragment alpha to 1 when
the fragment is not discarded, which prevents the "punch through"
effect. This is consistent with ThreeJS.
Fixes#4576.
Philip Rideout
e8d8080928
