Add PHP bindings for setting and getting the image resolution

Add PHP bindings for setting and getting the image resolution

We expose the image resolution related GD functionality to userland
by introducing `imageresolution()` as getter/setter. Given only the
image argument, it returns the current resolution as indexed array.
Given only a second argument, it sets the horizontal and vertical
resolution to this value. Given three arguments, it sets the horizontal
and vertical resolution to the given arguments, respectively.