Fix bug in ply conversion (shadowalpha)
Fix bug in ply conversion (shadowalpha)

"shadowalpha" textures weren't being correctly carried through.

mmp/pbrt-v3,nyue/pbrt-v3,mmp/pbrt-v3,Vertexwahn/pbrt-v3,nyue/pbrt-v3,nispaur/pbrt-v3,mmp/pbrt-v3,Vertexwahn/pbrt-v3,bssrdf/pbrt-v3,mwkm/pbrt-v3,shihchinw/pbrt-v3,shihchinw/pbrt-v3,mmp/pbrt-v3,ambakshi/pbrt-v3,ambakshi/pbrt-v3,bssrdf/pbrt-v3,mwkm/pbrt-v3,Vertexwahn/pbrt-v3,bssrdf/pbrt-v3,nispaur/pbrt-v3,shihchinw/pbrt-v3,nyue/pbrt-v3,nyue/pbrt-v3,shihchinw/pbrt-v3,ambakshi/pbrt-v3,nispaur/pbrt-v3,nispaur/pbrt-v3