`ImagePaintMode` `paint_bucket_fill` does a double colorspace conversion
atm. (which is a lossy process).
Since this is lossy, painting with the same color as was used for
filling can give slight differences (very noticable though if painting
e.g. bump maps).
For comparison, `ProjectionPaintMode` `paint_bucket_fill` (so in the 3D
viewport) does not suffer the same issue (it keeps track of both sRGB
brush color and a linear version of such color).
Currently `paint_2d_bucket_fill` expects linear space color values, so
for filling byte images, we first convert the brush color to linear
(`srgb_to_linearrgb_v3_v3`) then inside `paint_2d_bucket_fill`, we
convert back (`linearrgb_to_srgb_v3_v3`).
We can avoid the double conversion though, make `paint_2d_bucket_fill`
expect sRGB space color values and only convert to linear if we a
filling float images.
Pull Request: https://projects.blender.org/blender/blender/pulls/138540
a5db664d61