PostProcessing in A-Frame works only in desktop mode

[enzofrancescaHM]

Description of the bug

Followed the instructions in the wiki it is possible to integrate PostProcessing into A-Frame, everything work as expected, but when entering VR mode the screen is completely white.

To reproduce

Go to the example linked in the Wiki: https://jsfiddle.net/a38r5cb6/ and try to enter VR.
Same behaviour on Quest 3

Expected behavior

VR mode and desktop mode should be equal

[vanruesc]

Hi, that wiki page is quite old and proper VR support hasn't been implemented in postprocessing yet. VR hasn't been the focus of this project and implementing it isn't straight forward.

I have opened a ticket for gathering information about implementing VR to move this forward -- closing in favor of #677