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.
Updated OSL emission test scene.
Fix bug when using P in shaders with emission closures. Small refactors.
Formatting
Merge pull request #558 from est77/OSL_emission

OSL emission fixes
Merge branch 'master' of github.com:appleseedhq/appleseed into FrozenDisplay

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