[BREAKING] Only rewrite image paths for RTL-variant when files exist

[tobiasso85]

Only rewrite img path to img-RTL in RTL (right-to-left) CSS if image file is present in img-RTL folder.
The mirrored images need to be present on the filesystem. Only if they are present the img url path
is rewritten in the resulting RTL CSS.

Samples

Mirrored

filesystem:

• img/subfolder/control.png
• img-RTL/subfolder/control.png

CSS

.myclass {
  background: url(img/subfolder/control.png);
}

RTL CSS

.myclass {
  background: url(img-RTL/subfolder/control.png);
}

Not Mirrored

filesystem:

• img/subfolder/number.png

CSS

.myclass {
  background: url(img/subfolder/number.png);
}

RTL CSS

.myclass {
  background: url(img/subfolder/number.png);
}

Note: Neither absolute URLs nor absolute paths are rewritten (e.g. background: url(http://example.com/img/fg/hi.png); or background: url(/img/fg/hi.png);)