Don't use the stripe texture when we don't need it

IE 11 on Windows 8.1 Update 1 adds luminance texture support, so it's only needed for old versions of IE 10/11 and the Flash fallback w/ Stage3d now.

Partway through decoupling the stripey mode... need to finish:
 - use the striped mode fragment shader for Flash
 - include both shaders in JS and use the appropriate one at runtime