Fixed imgtool diff saving.
Fixed imgtool diff saving.

RGBSpectrum::FromRGB is a static function, not a mutator method, so we need to explicitly write into the diffImage array.
nyue/pbrt-v3,nyue/pbrt-v3,mmp/pbrt-v3,shihchinw/pbrt-v3,mmp/pbrt-v3,nyue/pbrt-v3,mmp/pbrt-v3,shihchinw/pbrt-v3,shihchinw/pbrt-v3,mmp/pbrt-v3,nyue/pbrt-v3,shihchinw/pbrt-v3