Terrain demo much extended

Our procedural terrain demo (see kambi_vrml_game_engine/examples/vrml/terrain in SVN) got a lot of improvements this week:

• Heterogeneous terrain (idea from Ken Musgrave) implemented, this makes more realistic terrain (smooth valleys, noisy mountains).
• Terrain is rendered with nice blended texture layers, normals are calculated for fake lighting, fog may be used, all by simple GLSL shader.
• Simple LOD approach for rendering is used, this is an ultra-dumbed-down version of geometry clipmaps. The way I simplified this is unfortunately painfully visible as LOD "popping" artifacts. But, hey, at least I can view really large terrain.
• You can test various noise interpolation methods, including Catmull-Rom splines.
• 2D noise can be blurred, which (may) improve the terrain look.
• You can switch camera to Walk mode.
• You can add a heighmap from grayscale image to generated terrain.
• Rendering uses VBOs for speed.

Finally, the programmers may be interested in my my notes and links about basic terrain generation methods.