Fix raytrace.rgen in ray_tracing__advance

[lukelmy]

According to ray_tracing_jitter_cam, we should write directly to the image if the frame number is equal to 0 in "raytrace.rgen".

However, there is an equal sign in "raytrace.rgen" of ray_tracing__advance, which may cause corruption.

[mklefrancois]

Thanks for reporting the issue. It works because when pcRay.frame ==0, a will be equal to 1.0F and the image will store only prd.hitValue. But to avoid any confusion and the unnecessary imageLoad, the code will be changed.