C4Landscape: Make GetPix and _GetPix const
Add ambient lighting

This introduces a new texture, an ambient light map, that is generated
automatically at the beginning of the round by the sky portion of the
landscape. This basically makes everything that is close to sky visible
by default.

The shaders have been adapted so that they deploy direction-independent
lighting for the ambient component, and the current (diffuse) behaviour
for the diffuse component. This makes the shaders use an additional
texture unit that represents the ambient light. We can think about merging
this information into the light texture, but the coordinate systems are
different at the moment, so this could be performed at the stage of light
texture generation.

For meshes, the ambient material is not actually used, but instead a
diffuse light from the front is used. This makes many meshes look more
interesting, maybe also because the ambient material setting of most
meshes are not set correctly at the moment.