gl_video: actually fix shader compilation on OSX

The previous attempt was missing some code paths, so there were still
shaders generated that triggered the shader compilation error. Fix it
instead by special handling USE_CONV in the shader.

By the way, the shader compiler did not accept:

    #if defined(USE_CONV) && (USE_CONV == ...)

In my opinion this should be perfectly fine, but it gives the same
error as before. So test USE_CONV separately with #ifndef.