(By calling Imf::setGlobalThreadCount() in InitPBRT().)

This noticeably speeds things up after rendering with high-resolution and
many-channel output images.

Fixes #159.

imgtool- Add DCI-P3 to the list of available colorspace options

There's no need to carry around a heavy ParameterDictionary here; getting
rid of it markedly improves Moana startup time.

This gets rid of a bunch of unnecessarily duplicated code...
(And removes a long-obsolete assumption that all area lights are
DiffuseAreaLights.)

First here is now SceneStateManager, which implements the ParserTarget
interface and takes care of handling graphics state management. As parts
of the scene are defined, it passes them along to a SceneProcessor, a
new interface that currently has a single implementation that now takes the
name "ParsedScene".

In time, this will make it easier to try asynchronous creation of scene objects
as parsing occurs. For now, it seems to give a ~5% reduction in startup time
for the Moana Island scene.

(Lots of files not generated from the book had gotten crusty.)

Add support for reading gzip scene files

No functional changes, but a little less redundancy now.
Also, fixed a few memory leaks.