Adds a new plugin that has elements that perform geometric
transformations to images. By geometric transformations I mean
that the operations are functions that given the output pixel
position, get the pixel position in the input image. This pixel
is then copied from input to output.

The gstgeometrictransform baseclass makes it easy to write
such elements. It boils down to write the mapping function
and exposing properties

Already added the first of the elements, 'pinch'. It's a common
effect in image editors, like gimp (distort -> pinch)