Add zoom support to SDL2 renderer

Checks to see if the environment supports the required features
and if so enables zoom.
Fix keybinding for zoom in.

The + key doesn't exist on US keyboards
Merge pull request #131 from TheCycoONE/sdl2_scale_fix

Added zooming to the SDL2 branch. Note that the scroll wheel is currenty not usable. Use either + or = to zoom in and - to zoom out.