Update Image_lib.php

Only wm_x_transp and wm_y_transp greater than 0,function "imagecolortransparent" with execution.
Because in wm_type = overlay, wm_x_transp = 0, wm_y_transp = 0, the image have partially transparent.
If you change the test to ``FALSE`` than you can have a simple way to ignore transparancy.

```
if($this->wm_x_transp !== FALSE) imagecolortransparent($wm_img, imagecolorat($wm_img, $this->wm_x_transp, $this->wm_y_transp));
```