Add build prerequisite steps for openSUSE.
Update requirement text to include AoE II: HD for asset conversion.
Don't store rendering mode in Texture

If this is a property of the texture itself, that fact should be stored
in the texture metadata. If it's a property of rendering, we should note
that when it's rendered, as we do after this commit.
Add AssetManager

AssetManager owns textures and is responsible for loading them, or
loading the new 'missing' texture if the file is unavailable.
Use AssetManager in Terrain
Implement Texture reloading
Use inotify to monitor for changed assets