Improve cursor_set_custom_image() method

[KoBeWi]

cursor_set_custom_image() has image in name, mentions image in the description, but the resource it expects is actually Texture2D. Moreover, the Texture2D is converted to Image inside the method and every DisplayServer implementation has pretty much the same code to do so...

This PR

• adds a common method to convert a Resource to cursor Image
• the method now takes either Texture2D or Image
• DisplayServers will use the new method instead of repeating the code
• clarified documentation

[Mickeon]

I am to assume this will require some GDExtension binding compatibility code of sorts.

[AThousandShips]

Compatibility for what? It doesn't change any bindings

[akien-mga]

Thanks!