SFML 2.6 compatibility

SFML 2.6 update

rust-sfml 0.21 requires SFML 2.6. It will not work with 2.5.

Added

• Added RcFont and RcText for reference counted text (see examples/rc-resources.rs)
• pub const DEFAULT for Vertex
• is_smooth and set_smooth methods for Font
• RenderWindow::recreate method to recreate a window with new parameters

Changed

• Update RcSprite and RcTexture documentation.

Fixed

• Missing #includes in CSFML
• Too restrictive lifetime for Music