Implement nicer basic terrain rendering for opengl

The widelands software renderer creates a nice looking terrain by exchanging some pixels on the borders of the triangles. This is done by direct pixel access. For opengl rendering there is no such blurring on the borders of the triangles.

For opengl a nice looking terrain should be rendered without accessing single pixles. This is about a basic terrain renderer which should run on most systems. So only common available OpenGL features should be used.