Fix compatibility with scssphp 1.4 (see #2547)

Description
-----------

| Q                | A
| -----------------| ---
| Fixed issues     | Fixes #2495

### ToDos

- [ ] Revert contao/conflicts@820074e once this got merged and released (4.9 and upstream).

Commits
-------

961abaa Set the file path to fix compatibility with scssphp 1.4

core-bundle/src/Resources/contao/library/Contao/Combiner.php

@@ -416,7 +416,7 @@ protected function handleScssLess($content, $arrFile)
 				$objCompiler->setSourceMap(Compiler::SOURCE_MAP_INLINE);
 			}
 
-			return $this->fixPaths($objCompiler->compile($content), $arrFile);
+			return $this->fixPaths($objCompiler->compile($content, $this->strRootDir . '/' . $arrFile['name']), $arrFile);
 		}
 
 		$strPath = \dirname($arrFile['name']);