Videobox
--------

This plugin crops or enlarges the image. It takes 4 values as input, a
top, bottom, left and right offset. Positive values will crop that much
pixels from the respective border of the image, negative values will add
that much pixels. When pixels are added, you can specify their color. 
Some predefined colors are usable with an enum property.

The plugin is alpha channel aware and will try to negotiate with a format
that supports alpha channels first. When alpha channel is active two
other properties, alpha and border_alpha can be used to set the alpha
values of the inner picture and the border respectively. an alpha value of
0.0 means total transparency, 1.0 is opaque.

The videobox plugin has many uses such as doing a mosaic of pictures, 
letterboxing video, cutting out pieces of video, picture in picture, etc..

TODO

- add enum to specify common aspect ratios/sizes and add borders/crop
