There's a lot of places in the code that assume macOS is never GLES, and
with the increasingly rotten native GL support on Apple platforms people
are switching to ANGLE. So we should make sure this use case works.
Well, no wonder it OOMs with 38 parallel jobs on ~10-20 GB RAM depending
on how the VMs are loaded, heh. Will revisit when I get rid of more STL,
this has to suffice for now.
On Ubuntu, because SwiftShader Vulkan doesn't compile on GCC 4 (or 5)
anymore, we use 4.8 on 16.04 for compile-test against libvulkan-dev and
18.04 for render-test with SwiftShader.
On Mac, because we want to work with both MoltenVK and Vulkan Loader, we
compile-test against MoltenVk in the generic "all build" and render-test
against SwiftShader on a new, Vulkan-specific build. That requires us to
supply our own loader as well -- apparently no such thing is on
Homebrew, which is kinda silly (doesn't anybody do real Vulkan work on
this crazy platform?!).