New PanZoom linear transform

This PR brings a `PanZoomTransform`, similar to `STTransform`, but lighter, restricted to 2D, and with a different transform order.

Also, this transform can be used like this:

```python

pz = PanZoomTransform()

# Declare that pan and zoom are uniform variables
pz.pan = Variable('uniform vec2 u_pan', (0, 0))
pz.zoom = Variable('uniform vec2 u_zoom', (1, 1))

# Update the uniform values efficiently and intuitively
pz.pan = (1., 0.)
pz.zoom *= 2.
```