testshade multithreading, and other minor fixes

Makes testshade operate with multiple threads! Fairly simple, by using the handy OIIO parallel_image template.

This is really helpful for two reasons: (1) it will help catching threading bugs early, because every testsuite run will be multithreaded; (2) it makes it easier to catch threading bottlenecks with test shaders.

Also some other minor enhancements of testshade & testrender, such as printing TextureSystem and ustring statistics when in debug or stats mode.

In the process, I added methods to TextureSystem that allow retrieval of the underlying TextureSystem or RendererServices pointers.