* This updates our assimp version to v5.2.1
* This is needed because the latest assimp doesn't compile without some
patches :-/ The patches required and applied are listed in the README
located in extern/assimp.
* Now shader recompilation happens only when it needs to. Hurray for faster compiles!
* Changes to constructors for ShaderSource to make it more explicit.
* The version of SPIRV-Cross is updated, and now the glslang version pulled correctly matches what SPIRV-Cross needs. This fixes the annoying "cannot parse built-ins" bug.