Bugfix: Prevent replacing string occurences of extension in filename

[rebeccahum]

Hi!

Any filenames with the extension in it are being stripped out from this line:

Aqua-Resizer/aq_resizer.php

Line 128 in 57fe7e7

$dst_rel_path = str_replace( '.' . $ext, '', $rel_path );

And thus, fails this check here:

Aqua-Resizer/aq_resizer.php

Line 136 in 57fe7e7

elseif ( file_exists( $destfilename ) && getimagesize( $destfilename ) ) {

E.g. /2019/01/filename.jpg-hello.jpg-test-name.jpg, where all occurrences of jpg become stripped out when only the actual extension at the end of it should be. This patch looks for the last occurrence of the extension at the end and strips it out by position instead.