`[RawVis]` Dark mode shouldn't affect raw JPEG/PNG images

[axelboc]

          I have added opaque support (not yet released) but I am not sure yet if I did it right. Is it expected that the colors in H5web are different from the original?

Originally posted by @Apollo3zehn in #1623 (comment)

[axelboc]

Easy fix, thanks for the report @Apollo3zehn

[axelboc]

The fix is now live on myHDF5