frozen display prototype.
Merge branch 'master' of github.com:appleseedhq/appleseed into FrozenDisplay
fixed compilation on Windows and many smaller problems.
fixed incorrect construction of renderer::ShadingRay objects after recent introduction of dtime.
cosmetics.
renamed renderer::ImageStack::get() to get_index().
renamed renderer/api/tracecontext.h to trace.h and added renderer/kernel/shading/shadingray.h to the list of public API headers.
frozen display mode is now optional (disabled by default). refactorings.
fixed assertion.
simplified and refactored the frozen display mechanism.
Merge branch 'master' of github.com:appleseedhq/appleseed into FrozenDisplay

Conflicts:
	src/appleseed/renderer/kernel/lighting/lightsampler.cpp