Refactored Mesh internals and formats.

-Changed how mesh data is organized, hoping to make it more efficient on Vulkan and GLES.
-Removed compression, it now always uses the most efficient format.
-Added support for custom arrays (up to 8 custom formats)
-Added support for 8 weights in skeleton data.
-Added a simple optional versioning system for imported assets, to reimport if binary is newer
-Fixes #43979 (I needed to test)

WARNING:

-NOT backwards compatible with previous 4.x-devel, will most likely never be, but it will force reimport scenes due to version change.
-NOT backwards compatible with 3.x scenes, this will be eventually re-added.
-Skeletons not working any longer, will fix in next PR.

core/io/resource_importer.h

@@ -102,6 +102,7 @@ public:
 	virtual String get_resource_type() const = 0;
 	virtual float get_priority() const { return 1.0; }
 	virtual int get_import_order() const { return 0; }
+	virtual int get_format_version() const { return 0; }
 
 	struct ImportOption {
 		PropertyInfo option;