Adding a background image that is not affected by the Default Rendering Pipeline post processes

@Evgeni_Popov @sebavan Update! I have found some breakage with recent updates to babylon with this particular technique and we’re experiencing this after updating to ^5.53.1. (^5.21.0 was working for us) See PG example:

this happens when we set the HDR mode in the DefaultRenderingPipeline to true

We do need HDR PP , we cant really rely on LDR.

Is this a bug or is there a better way to do this?