core/colorspace: use gamma22 for sRGB content

sRGB content is made to be encoded with the sRGB piece-wise transfer function,
and to be decoded by displays with the gamma 2.2 transfer function.
When KWin has a display in sRGB mode, this doesn't make a difference - it
decodes with sRGB and encodes with sRGB, so there's effectively no transformation.
When the display is in PQ mode though, KWin uses the sRGB inverse EOTF for
decoding sRGB content, but not the sRGB EOTF for encoding it again.

To fix this, this commit changes KWin to use the gamma 2.2 EOTF and inverse
EOTF for untagged content. That's not technically correct for sRGB screenshots,
where we'd have to use the piecewise sRGB EOTF, but that's a problem that
can be solved in the future.