qt: introduce fragment-shader-level postprocessing in dual kawase blur

Currently supporting tinting, noise, and exclusion. These are to be used by the frosted glass effect.

Co-authored-by: Pierre Lamot pierre@videolabs.io

This also reverts the frosted glass look back to the time when we used an additional layer to have exclusion and tint (988b7bb2). Noise-wise, it should not have any changes.