Mat4::frustum_{lh,rh{,_gl}}

[valaphee]

Solves #441

[valaphee]

perspective could be built on top of frustum, but would incur some overhead, and the test currently works by comparing perspective, with frustum.

Could change the test to also check projections.

[bitshifter]

Thanks for taking this on!

It looks like clippy is failing because perspective is unused in the tests you've added, from your comment above it sounded like you were intending to use that value in the test though?

As for perspective being constructed from frustum, I don't have a strong opinion. Re-use is nice when it makes sense and a little perf hit isn't that big a deal IMO in these as I think they're not usually frequently called in hot code, but what might that look like?