Fixed leftover cosine term, reference images.

        - Fixed left-over cosine term in final accumulator expression.
        - Re-rendered reference images, updated test scenes.
Merge pull request #414 from est77/master

Fixed Imath cmake library names on linux.
tweaked formatting.
minor code tweak.
Fixed potential negative pdf values.

        - Fixed potential negative pdf values when evaluating the BRDF
          response, one of the cos(phi_diff) factors was not absolute.
minor code tweak.
minor code tweak.
added renderer::ColorSource::get_color_entity().
bump version id of object instance when modified.
Added cumulated transform sequence, to make it easier to implement object coordinate systems.
Save accumulated transform sequence in assembly instances.
fixed tumble/track/dolly speed in camera controller.

this partially reverts changes introduced in #378.
cosmetics.
removed unused rply library.
Merge remote-tracking branch 'upstream/master' into OSL_objectspace
added an implementation of SipHash.
added renderer::compute_signature() to compute the signature of an entity.
added renderer::Source::compute_signature() to compute the signature of a source.
intersection filters are now cached.
fixed crash when a project is reloaded during rendering.