Added multi-sampling option in TextureFramebuffer.

- TextureFrameBuffer can now try to allocate a multi-sample render
target both for color and depth.
- Surface now defines an accessor indicating the number of samples if
multisampling is activated.
- Texture can now create mutli-sampled render targets.
- SGL now provides optional multisampling, with a getter allowing to
now if it is possible, as ES 2.0 does not support it (but 3.0 does).
- Clarified usage of GLText.