backends/drm: do blending in gamma 2.2 space with color management too
This matches what we do without color management, and is more what users and app developers expect. Going with linear blending before was mostly from it being more "correct" / how physical blending of light works, but that doesn't really matter when existing software expects it to behave differently.
Storing the blending result in gamma 2.2 encoding also allows us to drop the shadow buffer(s) from 16bpc to 10bpc, which needs less power and doesn't have special driver bugs. That as well as removing the shadow buffer completely in HDR mode on not super old hardware (by using KMS to do the gamma 2.2 -> PQ conversion) will be done in future MRs though.
With the same blur settings and the Konsole background configured to be 35% transparent in both cases, the difference looks like this:
| Before | After |
|---|---|
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