Add bvhvec4slice to Build()/BuildHQ()

[DavidPeicho]

Sorry for the drive by cleanup, I have a plugin to remove trailing whitespaces, didn't realize since it autoruns on save :) If you want I can eventually revert those changes.

Would be great to add bound checks to the slice in debug mode. Haven't done that yet. Should we add a DEBUG define?

[jbikker]

That looks good! For the new 'fast builder' this will be trivial to include as well, and for the AVX / NEON builders the slices can simply be union'ed with __m128 or the NEON equivalent.
Don't worry about the spaces, I will see if Visual Studio can delete those automatically as well. I use a code standard template, but deleting trailing spaces appears to be missing from it.
This does bring up two follow-up 'issues':

1. The bvhvec4 requirement is really just there for the AVX and NEON builders. Perhaps this will make more sense to the user if it is exclusively enforced for those; ::Build can then simply take bvhvec3 slices.
2. I see your nice auto-documentation comment and realize I need this everywhere now. :)
   Is it proper etiquette on Github if I create issues like that for myself (or others, if they beat me to it, I guess)?

[DavidPeicho]

1. Makes perfect sense yes. We should just keep a separate API for those, and indeed accept bvhvec3 then for Build.
2. Don't know if there is a "proper" way, whatever you like the most. I like to create issues, label them, and assign them yes

if we create slices for other types, like bvhvec3, we should think about just creating a template slice type. I know you probably have converns about template, but we could explicitly instantiate them for the types we use (vec3/vec4?)