Merge height and meta data into a single texture

[kvark]

Currently they live in separate textures and even have different types (height is normalized, meta is not). This is similar to the original Vangers code. However, the actual usage always involves scanning both of them, since the height doesn't make sense without the meta anyway. Thus, merging them into [u8; 2] would allow us to reduce the number of texture samples and improve the cache efficiency.