Fixed leftover cosine term, reference images.

        - Fixed left-over cosine term in final accumulator expression.
        - Re-rendered reference images, updated test scenes.
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.
removed unused rply library.
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.
improved entity signature mechanism.
removed renderer::TextureInstance::set_transform().

In general, entities are immutable: once created they cannot be modified. There are a few exceptions, for instance materials can be assigned to existing object instances.
minor bug fix (triggered an assertion in debug but worked anyway).
removed redundant debug messages.
the uniform pixel renderer now uses the actual number of samples requested when decorrelation is enabled.
formatting.
Updated scenes, added reference images.

        - Removed old reference images.
        - Updated ONW scenes and reference images.
small code tweak.
tweak warning message.
added transparency threshold to SPPM parameters and (really) disabled redundant log messages.

fixes commit 512894b95ffe07ae8c6f43a8145082d7afb95fda ("removed redundant debug messages.")
defined FMT_UINT64_HEX preprocessor symbol and fixed compilation on OS X with clang.
minor code tweaks.