Add Gltf ModelTemplate

[dragbone]

Summary:

• Add release checks and bool to enabled/disable releasing of geometry to Mesh
• Move Model generation from GltfFile to new ModelTemplate
• Cache geometry and shader in ModelTemplate

Note: Mesh hasn't checked if its geometry has been released until now. The added release checks in Mesh will probably break some cases that "worked" with async rendering but would probably have failed on non-async backends.