Fixes a number of warnings found under GCC 4.8, Ubuntu13.10

Primarily signed->unsigned comparisons.
Fixes crash when SDL_JoystickName() returns NULL.

(Was happening for me on Ubuntu, with a wireless Xbox 360 controller
adapter plugged in, but with the controller turned off.  This probably
needs a lot more testing)
Fixes mouse wheel speed on Linux builds.
Merge branch 'master' into dynamic_material
Renderer now correctly handles render-to-target.

(This was an oversight during a recent refactoring of code.  Just hadn't
connected all the pieces correctly)
Adds ability to 'disable' a scene.

When scenes are disabled, they won't be drawn automatically.  (You may
still draw them explicitly using functions like "DrawSceneToTarget()")

(This sort of hack is making me feel more and more like I need to
refactor VectorStorm's scene rendering code to something more flexible.
Originally, I had been aiming for ultimate simplicity, but as I'm making
more and more serious projects, this simple setup just isn't giving me
the power I need)