Rather than running the runloop forever, have the API expose an incremental runloop. This allows clients to do other processing if they need it.
This allows for removing the odd construction of having knowledge of GTK event handling built into the library even though nothing in the library uses it; instead runner applications that use GTK plugins (such as FDE's testbed) can do that processing at the application level instead.
Simplifies the build and runtime requirements for the Linux shell.
Since the engine does GL extension lookup manually anway, an extension
loader library isn't necessary.
Changes include:
- File structure
- Header guards
- Include paths
- Namespaces
- Integration with the engine's GN build
- Conversion from jsoncpp to rapidjson
- Style and clang-format adjustment to match engine repository
