`convert_to_jpeg!` is only called if `convert_png_to_jpeg?` and/or `should_alter_quality?` is true.
`convert_png_to_jpeg?` can be disabled by setting `SiteSetting.png_to_jpg_quality` to 100.
However, `should_alter_quality?` could be true if `SiteSetting.recompress_original_jpg_quality` was lower than the quality of the uploaded file, regardless of file type.
This commits changes `should_alter_quality?` so that uploaded png files will use the `SiteSetting.png_to_jpg_quality` value, rather than ``SiteSetting.recompress_original_jpg_quality` value.
