Encapsulate runtime creation.
- Makes it easier to change down the line (and change is coming)
- Allows making g_ScriptRuntime thread-local easily.
- Remove ParentRuntime, which is not used at the moment.
Part of the SM52 migration, stage: SM45 compatible.
Patch by: Itms
Tested By: Freagarach
Refs #4893
Differential Revision: https://code.wildfiregames.com/D3087