Rework Mesh handling on scene importing.

-Reworked how meshes are treated by importer by using EditorSceneImporterMesh and EditorSceneImporterMeshNode. Instead of Mesh and MeshInstance, this allows more efficient processing of meshes before they are actually registered in the RenderingServer.
-Integrated MeshOptimizer
-Reworked internals of SurfaceTool to use arrays, making it more performant and easy to run optimizatons on.

modules/meshoptimizer/config.py

@@ -0,0 +1,7 @@
+def can_build(env, platform):
+    # Having this on release by default, it's small and a lot of users like to do procedural stuff
+    return True
+
+
+def configure(env):
+    pass