This removes the need for per mipmap scalling factor and trilinear interpolation
issues. We pad the texture so that all mipmaps have pixels in the next mip.
This simplifies the downsampling shader too.
This also change the SSR radiance buffer as well in the same fashion.
This effectivly reduce firefly bleeding all over the place.
We still need the clamp in the resolve pass because the level 0 has not been clamped.
NOTE: I did not clamped each sample individually for performance BUT I did not profile it to know how much it cost.
This was cause by some post process not always sampling the highest mipmap.
But if there is no need for mipmapping (i.e. no SSR) these levels will be undefined.
So forcing all Post FX shader to sample level 0 fix this.
