Incorrect face-varying interpolation for FVAR_LINEAR_CORNERS_PLUS modes

[manuelkNVDA]

Wide limit samples divergence in face 253 for catmark_left_handed and catmark_right_handed test shapes.

This test shape is relatively complex, it would be nice to have a much simpler case.

Batch face-varying (linear corners-plus1 interpolation) {
        Evaluation Options:
                 -vtx           (vertex bnd interp)     = 'default'
                 -fvar          (fvar bnd interp)       = 'override corners plus1'
                 -skipvtx       (ignore vertex eval)    = true
                 -d1            (eval 1st deriv)        = true
                 -d2            (eval 2nd deriv)        = false
                 -uv            (eval UVs)              = true
        Results: pass:107 / known fail:0 / fail:2 (109/109)
        'catmark_lefthanded' (1.063608s):
                 UV diffs:     1 faces, max delta UV = 0.0409195
        'catmark_righthanded' (1.124277s):
                 UV diffs:     1 faces, max delta UV = 0.0409195
}
Batch face-varying (linear corners-plus2 interpolation) {
        Evaluation Options:
                 -vtx           (vertex bnd interp)     = 'default'
                 -fvar          (fvar bnd interp)       = 'override corners plus2'
                 -skipvtx       (ignore vertex eval)    = true
                 -d1            (eval 1st deriv)        = true
                 -d2            (eval 2nd deriv)        = false
                 -uv            (eval UVs)              = true
        Results: pass:107 / known fail:0 / fail:2 (109/109)
        'catmark_lefthanded' (1.100888s):
                 UV diffs:     1 faces, max delta UV = 0.0188207
        'catmark_righthanded' (1.104047s):
                 UV diffs:     1 faces, max delta UV = 0.0188207
}