GeometryShaderManager: Set stereo parameters in a SetConstants() call.

Doing it in SetProjectionChanged() is too early because the projection type is not set yet.
I think I'll remove the former check, does it really cost that many cycles to calculate the offsets below that we need to optimize them out when stereoscopy is disabled?

Also, there's a potential bug here where stereoscopy is enabled but the projection did not yet change to set the stereo parameters. That one can be solved by moving the stereo mode check up and not reset `s_projection_changed` until stereoscopy is enabled.