Fix bug in ImageFilm::GetSampleExtent() 
Fix bug in ImageFilm::GetSampleExtent() 

(Reported by Christian Lessig).

When the fractional component of the filter function's width is exactly
0.5, this function would end up requesting an unnecessary row and column
of samples along the edges of the image.  This is now fixed.

patwonder/pbrt-v2-skin,patwonder/pbrt-v2-skin,patwonder/pbrt-v2-skin,patwonder/pbrt-v2-skin,patwonder/pbrt-v2-skin