[WIP] Metal Effect Support

[TheSpydog]

Very early draft of an Effects interface for Metal. It's mostly copy-pasted from mojoshader_opengl.c with a few significant changes:

• mtlCompileEffect concatenates all the MojoShader shader output into a single string, then compiles that conglomerate into a MTLLibrary. One Effect <=> One Metal Library.
• Since Metal has no concept of a "shader program" like GL does, we store pointers to the MTLLibrary, the vertex MTLFunction, and the fragment MTLFunction associated with each effect pass.
• mtlDeleteEffect is a total no-op currently. Putting this off since we'll have to deal with Objective C's funky memory management...

[TheSpydog]

Superseded by #13 because I accidentally nuked the repo this was pulling from. Whoops.