Lighting

[kvark]

We could greatly enhance the looks and enrich the volume by adding simple phong lighting. Models already come with normals and solid colors, so these are trivial to light. Problem comes from the terrain:

• it doesn't have any normals, so we'll need to compute them dynamically
• we need to ray-cast again when drawing into the shadow map
• there is already some non-trivial lighting computation in the shader that needs to be revised for compatibility

[kvark]

We could go 2 ways about the model lighting:

1. just do it on our own with a Phong model
2. follow the existing code:

    unsigned char color_offset = COLORS_VALUE_TABLE[color_id*2];
    unsigned char color_shift = COLORS_VALUE_TABLE[color_id*2 + 1];
        unsigned c = color_offset + (((unsigned)normals[0] -> I_8 + normals[1] -> I_8 + normals[2] -> I_8) >> color_shift)/3;

where I_8 is computed by fast_calc_intensity()