[BUGFIX] Fix path resolution in t3lib_compressor::compressCssFile()

This change makes compressCssFile() also use this helper, as it was forgotten in the
first place.

Change-Id: Id394f9a67e6222f7663e719ddce602d5ada2d10c
Resolves: #M18035
Reviewed-on: http://review.typo3.org/1176
Reviewed-by: Steffen Gebert
Tested-by: Steffen Gebert

t3lib/class.t3lib_compressor.php

@@ -225,7 +225,7 @@ class t3lib_Compressor {
 	 */
 	public function compressCssFile($filename) {
 			// generate the unique name of the file
-		$filenameAbsolute = t3lib_div::resolveBackPath(PATH_typo3 . substr($filename, strlen($GLOBALS['BACK_PATH'])));
+		$filenameAbsolute = t3lib_div::resolveBackPath(PATH_typo3 . $this->getFilenameFromMainDir($filename));
 		$unique = $filenameAbsolute . filemtime($filenameAbsolute) . filesize($filenameAbsolute);
 
 		$pathinfo = pathinfo($filename);