Require SM 5.3 and up (half floats...)
Print a more useful error when the GPU's shader model is insufficient.

The PMJ02BNSampler now takes a "seed" parameter.
Improved dimension->pmjInstance indexing (issue #36, via @Andrew-Helmer)

Folded HaltonPixelIndexer back into HaltonSampler, among other small changes.

Includes a fix to a bogus DCHECK in the Curve intersection code.

Also adds a comparison to the tHit of any previous curve intersection to be
sure that we actually return the closest intersection.

Remove variance from EXR metadata
Change GBufferFilm to compute per-RGB-component variances

Note: the Film's "scale" parameter has been removed, as it is redundant given "iso" now.