DisplayListBuilder sets the opacity peephole flag despite conflicting saveLayer properties

[flar]

Currently the flag in the SaveLayer command that is passed to dispatchers only really indicates that the contents are compatible, but the presence of a backdrop filter or if the blend attributes of the SaveLayer blit are not SrcOver are also incompatible with doing the optimization. The DL should include those properties in the flag to reduce added tests in the dispatchers for these same conditions...