Substituting metal shaders uses wrong entrypoint

A bit of background: I'm trying to use a shader with "dual-source" blending mode. The MSL code as produced by QSB is not correct, because Metal expects color(0), index(1) for the second output, instead of color(1). I suppose this is an upstream issue with SPIRV-cross.  Apparently, my shader was written wrong. The issue described below is still valid though.

I wanted to replace the autogenerated MSL shader with a corrected version (as described in the manual). When replacement shaders are provided, QSB uses a hardcoded "main" entrypoint, which isn't compatible with Metal.