Added new wrappers for render & texture functions.

- Render: render-copy, render-present
- Texture: update-texture
Merge pull request #31 from katco-/more-wrappers

Added new wrappers for render & texture functions.
Use &body on WITH-EVENT-LOOP for indentation semantics
Merge branch 'master' of github.com:lispgames/cl-sdl2
Make INDEX optional to CREATE-RENDERER; implement WITH-RENDERER.