Mobile support

[vorg]

Historically mobile support in pex-renderer was limited to "it worked on my iPhone7 two months ago" . Given our recent projects and their struggles with cross platform compatibility it would be good to discuss possible approaches.

1. Switch to mediump (Use getShaderPrecisionFormat to check if highp precision is available in shader #122) and test on iPhone7+ and flagship androids from 2018+ (list please)
2. Add proper support to unlit shader (currently enabled by setting roughness and metallic to null)
3. Implement material techniques support (game engines have fallback shaders for ultra / high / mid / low quality settings)
4. Add simpler lighting models like per vertex lamber diffuse, simple phong etc although how those would interact with PBR features indirect lighting, area lights is currently not known.