Cycles: add new Spectrum and PackedSpectrum types

These replace float3 and packed_float3 in various places in the kernel where a
spectral color representation will be used in the future. That representation
will require more than 3 channels and conversion to from/RGB. The kernel code
was refactored to remove the assumption that Spectrum and RGB colors are the
same thing.

There are no functional changes, Spectrum is still a float3 and the conversion
functions are no-ops.

Differential Revision: https://developer.blender.org/D15535

.clang-format

@@ -266,6 +266,7 @@ ForEachMacros:
   - MAP_SLOT_PROBING_BEGIN
   - VECTOR_SET_SLOT_PROBING_BEGIN
   - WL_ARRAY_FOR_EACH
+  - FOREACH_SPECTRUM_CHANNEL
 
 StatementMacros:
   - PyObject_HEAD