Support heat volume attribute
Support heat volume attribute

Similar to velocity, it was kind of supported by the mesh manager but
was missing a code in BlenderSession to get actual values.

In Cycles Heat is an attribute which goes from -1 to 1, where -1 is
the coldest ever temperature, 1 is the hottest ever one.

tangent-opensource/coreBlackbird,tangent-opensource/coreBlackbird,tangent-opensource/coreBlackbird