A new TextLayer allow one pass optimized text rendering.

- The TextLayer handles a set of fonts and associated text items to
render.
- It has a cache of text items.
- Also added dispose() operation in meshes to release GL memory.
- Added an orthographicPixels() method in Space to push a view in
pixels.
- The text shader is now stored in GLFont, not in GLString.
- The GLFont now has a beginRender() and endRender() to prepare for
lots of rendering with the same font.
- GLString has a corresponding set of render() methods assuming
beginRender() and endRender() has been called in GLFont.
- GLString now have length method ! And a maxLength method for capacity.