Adds support for specifying a custom entry point function name when creating compute shaders

[mvaligursky]

• default entry point is 'main', this allows to specify a custom one. It allows multiple entry points in a single shader module.

Usage:

new pc.Shader(device, {
    shaderLanguage: pc.SHADERLANGUAGE_WGSL,
    cshader: wgslCode,
    computeEntryPoint: 'myEntryPoint', // defaults to 'main' if not specified
    computeBindGroupFormat: ...
});