Allow to create materials,textures, models in threads.

[brainrom]

For future complete network support it'll be very useful, because now any attempts to load images over network (enet and yet unofficial HTTP) as textures results in massive lags when loading.

[bjornbytes]

Right now you can only create ModelData/TextureData in threads, which is only half the story. With Vulkan it should be possible to do GPU stuff in other threads.