math/mathwfd.h forward declares all {mat|vec}{2|3|4}<> classes,
which allows us to remove their respective #include in a lot of
our public headers.
Our math headers are full of templates, so this should help build times
a bit.
Also we want to keep the public headers as minimalist as possible.
If spherical harmonics are not specified in IndirectLight (i.e.
omitted entirely), we instead use the roughness==1 LOD in the shader.
This produce much better results with absolute HDR environment, and
generally works better more often.
This is now the preferred solution.
For cases where spherical harmonicas are better or preferred, they
can simply be supplied as before.
This adds a utility function on IndirectLight populate the reflection
map from an environment at runtime. This performs some processing
similar to cmgen, albeit at a reduced quality.
