Unsafe usage of `Mod_LoadModel` function

[a1batross]

One of the common functions used to load game assets is Mod_LoadModel which guesses model type and passes raw buffer to the appropriate loader.

However, this function doesn't account for any boundaries check, thus can be easily tricked into buffer overflow, and due to it's complexity it can't be patched right away either.

So let's break it down to separate, smaller tasks:

• mod_studio loader
• mod_sprite loader
• mod_alias loader
• mod_brush loader
• PhysicAPI/RenderAPI extension

Thanks to @veygax on bringing this up.