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