[BUGFIX] Compressor resolves dots in filenames correctly

Method getFilenameFromMainDir() tries to resolve a given file path
to a path relative to the document root and takes care of file
existance.

Sometimes it is fed with a path like '../path/to/file' and then
prepends it with the document root to call an is_file() on it. The
constructed path is then '/path/to/doc/root/typo3/../path/to/file'. If
now 'root' is a symlink to some sub structure itself, is_file()
will fail, because it does not solve the '..' part correctly anymore.

Using resolveBackPath() on the path before feeding it to is_file()
transforms the path to '/path/to/doc/root/path/to/file'. So the dots
are resolved and is_file is happy.

Change-Id: I20b4f4bbea695aeb02e3d92469236bc63cd05d97
Resolves: #52926
Releases: 6.2, 6.1, 6.0, 4.7
Reviewed-on: https://review.typo3.org/25010
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters