Implement model viewer mode, handy for debugging and in future, model creation

[hogsy]

Not sure on the best way to introduce this at the moment, maybe add it under tools? And then it switches the engine state to a specific mode for viewing models... Blergh!

---

• [x] See if we can render the model into a framebuffer and display the texture in an ImGui window
• [x] Allow multiple models to be open at the same time
• [x] Cache models into memory and ensure those are loaded through a 'LoadCachedModel' function which will search for a cached model (model viewer shouldn't use this method and should load it's own instance)
• [x] Refine interface with additional controls (like a list of loaded textures which then opens the texture viewer, etc.)
• [x] Allow the model to be pulled around in the viewport
• [x] Search for all supported model formats (at the moment it only searches for VTX)
• [x] Placeholder animation selection (needs to wait until animation implementation is actually done)
• [x] Option to make the model viewer fullscreen

[hogsy]

One last thing to sort and then this is complete, yay.

[hogsy]

Done!