using linear + binary search.

OpenGL_vertex:
* bufferize a duplicate calcul
* Factorize vertexes

This reverts commit 148fffb0.

OpenGL_vertex:
* bufferize a duplicate calcul
* Factorize vertexes

Skip shading for lightsources and top of the nodes.
Fixes liquid sources and flowing surfaces having different brightness.

Pass drawtype and material type to shaders.
Move shaders generation to startup only.
Allow assign shaders per tile.
Initial code to support water surface shader.

Parallax mapping with slope information.
Overriding normal maps.

If USE_CURL is set, it also downloads a list from a remote server.
The url of this list is configurable in minetest.conf using the setting "serverlist_url"
The local list of favorite servers is saved in client/serverlist/filename
filename is also configureable using the setting "serverlist_file"