Now it's much more usable, you are forced to use a hosted
shader compiler on a platform that needs it (for example, iOS)
and now CMake will error when it's missing. Now every
platform is very specific on which languages it needs to
be translated to, and whether a hosted compiler
is needed. No more manually copying over shaders!
* 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.
