RSM: add support for reflective shadow maps generation + GI based on RSM

[Popov72]

I wasn't sure where to add the files / how to name them (as well as the classes)...
Please let me know if I need to change anything in this regard.

[bjsplat]

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[bjsplat]

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[sebavan]

It looks amazing to me :-)

Could you add the reference you found in the shaders and the main class so that if we need to refer to it later on it is easier to find the technique details ?

Should we update the DOF kernel blur by the bilateral one here or the opposite ? It looks like we have two shaders with the same intent ?