Fix difference image computation in exrdiff
Fix difference image computation in exrdiff

It was broken when alpha was dropped from pixels returned from the image reading routines.

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