#1 gl.Extension string indexer

[pet_cz]

Add gl.Extension string indexer to allow simple testing of predefined list of extensions. E.g.

~~~~C#:::
string[] neededExtensions = new string[] { "ARB_multitexture", "ARB_vertex_buffer_object" }
...
foreach(string ext in neededExtensions)
if(!gl.Extension[ext]) throw new Exception(ext + " is not supported");
~~~~

[pet_cz]

Added bool gl.Extension.isSupported(string)