change projector_scheme_name variable from window to global

The projector_scheme_name variable doesn't work for me when it is a window variable.
I get the following:
Error detected while processing function ToggleProjectorMode:
line    1:
E121: Undefined variable: w:projector_scheme_name
E15: Invalid expression: (g:colors_name != w:projector_scheme_name) 

This PR changes it to a global variable, which works for me. Perhaps it's because I'm using vundle?