Added watermark text and image filters

Added filters : image watermark (wmi) and text watermark (wmt)
Using as long as possible phpthumb parameters...

Text watermark
fltr[]= wmt|<text>|<fontsize>|<position>|<font color>|<font file>|<opacity>|<edge>|<angle=not used !>|<bgcolor>|<bgopacity>

Image watermark
fltr[]=wmi|<imagefile>|<position>|<opacity=not used!>|<rotation>