Small bloom render pass optimization

[mvaligursky]

The bloom uses a series of downscaling and upscaling render passes to blur.
Instead of executing more costly 13-samples downscaling at all time, use just a single sample box filter for downscaling from full to half res, which is the most costly pass. No visible difference to the quality.

Cost of 4k rendering of Post-Processing example on apple m1 max:
before: 4.4ms
now: 4.2ms