Assess support for `EXT_mesh_primitive_restart`

[j9liu]

EXT_mesh_primitive_restart is a recent extension that is meant to reduce draw calls for continuous drawing modes (e.g., line strip or loop) by batching primitives together. The max index value (which is typically illegal) is otherwise used to separate primitives from each other.

Support for "primitive restart" varies across graphics APIs, so it's not clear whether or not our supported runtimes can really benefit from this. But ultimately, we could just have a setting that toggles the batching behavior in Native if we encounter the extension.

• Spec
• CesiumJS implementation