New text system in addition of strings.

- GLText allows more complex text compositions than GLString.
- Mesh has finally been updated with the elementsPerPrimitive method.
- This allows to finally add the draw(count) method that draws a mesh
only partially, by indicating how many primitive of the mesh have to be
drawn.
- Now VertexArray and Texture avoid to rebind if already bound (gains
lots of time !).