added optional feature for rewriting CSS images to output-fol...

[schmunk42]

...der on package compression.

Experimental! See also yiisoft/yii#1033 - how do you work around the images problem?

[mikehaertl]

Please rename to copyCssAssets. I'd also change the docs to:

Whether to copy CSS assets from their source location to the package output directory. Note, that this
feature is considered experimental. It will copy all files from the CSS files' source directory to the
package output directory which may not always lead to useful results.

[schmunk42]

I found a much better solution, IMHO. Please let me know what you think.

tl;dr
Basically don't copy anything, but (optionally) rewrite CSS file URLs relative to your webroot.

PackageCompressor ~line 149

            foreach(array_keys($this->cssFiles) as $file) {
                $f = $basePath.$file;
                if ($this->rewriteCssUris) {
                    file_put_contents($f, Minify_CSS_UriRewriter::rewrite(file_get_contents($f), dirname($f), $basePath));
                }
                $files[] = $f;
            }

This would replace the previous PR.

Background:
I ran into issue with YiiBooster, which contains relative URLs with ../fonts. The current proposal is unable to catch these cases and moreover the files are all already available in a public location.
So I seemed the most natural solution to me to reuse these files in the final CSS.

Works fine so far from a virtual host, I had an issue with running it from a sub-path, but that should be fixable.

PS: This would also add one composer dependency.

[edit]
sample output:
url(/assets/7dd6006e/img/ui-bg_glass_75_ffffff_1x400.png)