Compile shaders when building streamer

[barnabwhy]

Adds build step to compile shaders.

Lists of shaders to compile are in alvr/server/build_shaders.rs Shaders are in alvr/server/cpp/<platform>/shader/

Uses shaderc for linux shader compilation and fxc for windows shader compilation. Works with including from the same folder in shader files. Only does includes with 1 depth for HLSL since D3DCompile is horrible :3