Multi-Entry Point SPIR-V Shaders - Removal of IGPUShader and ICPUShader

What's Changed

There's now only a single IShader.

Shader Stage, and Specialization Info is being provided directly as Pipeline Creation Parameters.

This means that the SPIR-V each shader gets its capabilities and extensions trimmed based on the entry points used by a single pipeline.

Aggressive dead code elimination SPIR-V optimization is necessary for this to function.

Full Changelog: v0.6.2-alpha2...v0.7.0-alpha1