Determine good default quality for saving

[jakar]

Right now, the default quality is arbitrarily set to 50. Since very little effort was put into that particular decision, it is probably sub-optimal.

However, looking at libheif source, that value does map to crf=25 in x265, which seems to be a good balance. So maybe I picked a good default by sheer luck.

An opinion from someone more knowledgeable would be helpful.