Improve resource post-loading

[lannior]

Currently models and textures rely on the graphics backend to be loaded. That's why when you load a texture, it doesn't actually get filled with valid information until the graphics backend loads.

Currently there is a lot of boilerplate for resource post-loading and I'd like to reduce that, perhaps by introducing an event system to call the post load methods automatically and/or making a common type for this kind of resource.

Currently only models and textures need to be post-loaded, but more things such as shaders are going to be underway soon. So this issue needs to be tackled earlier rather than later.

Model post-loading
Texture post-loading
Running resource post-load methods