MicroSplat - URP 2019 Support

Adds support for Unity's Universal Render Pipeline. Unity's new Scriptable Render Pipeline requires a full rewrite of all shaders for each SRP. This module adds support for targeting the Universal Render Pipeline (2019.4LTS with 7.21), and works for all MicroSplat modules. Render Baking to textures is currently not supported. Requires MicroSplat Core. URP supported version is 7.21 on Unity 2019.4LTE.