This introduce some little artifacts on the border of edges because some pixel with very low opacity does not get discarded and then occlude the face rendered behind if it has not been drawn yet.
To fix this. I added an offset in the geometry shader for the edge fixup. This make the artifact only visible on the border of the object if there is a very dense wire region. It's only visible in edge select mode since vertex and face center also hides the artifacts.
We can enable this only if AA is enabled but for now it's always enabled.
Shader stages need to agree about interpolation qualifiers. Apparently implicit smooth (the default) and explicit smooth are considered different by some GLSL compilers. Found by @letterrip on Linux + Intel.
Follow-up to 941e739d70
