gltransformation: implement pivot point for rotation and scale

gltransformation: implement pivot point for rotation and scale

https://bugzilla.gnome.org/show_bug.cgi?id=744763

Add a pivot vector for setting the origin of rotations and scales.

With the pivot point the rotation and scale operations can have
different origins. This adds the ability to rotate around different points.
Currently the default (0, 0) pivot point is possible,
a rotation around the center, and zooming into and out of the center.

With an pivot point this is optional.
I defined the following image coordinates for the pivot point:

	 (-1,1) ------------------------- (1,1)
		|			|
		|			|
		|			|
		|	  (0,0)		|
		|			|
		|			|
		|			|
	(-1,-1) ------------------------- (1,-1)

Example:

	Rotate the video at the bottom left corner

	gst-launch-1.0 videotestsrc \
			! gltransformation \
		 		scale-x=0.5 \
				scale-y=0.5 \
				rotation-z=25.0 \
				pivot-x=-1.0 \
				pivot-y=-1.0 \
			! glimagesink

The pivot-z option defines the pivot point in 3D space.
This only affects rotation, since we have no Z data to scale.
With this option a video can be rotated around a point in 3D space.

Example:

	Rotate around point behind the video:

    	gst-launch-1.0 videotestsrc \
			! gltransformation \
				rotation-x=10.0 \
				pivot-z=-4.0 \
			! glimagesink