Adds a prepare function to make subclasses precalculate values
that will be used throughout the mapping functions.
Also adds a missing cleanup to fix a memleak
Renames some variables and adds a minimum doc to the
mapping function for a little clarity.
Also uses gstvideo functions for the row and pixel strides
instead of hardcoded values
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)
