Add viewport_x, viewport_y, viewport_width, and viewport_height counters

[flarn2006]

trafficstars

Since there seems to be no way to read (only to set) the viewport position and size from Robotic, I went ahead and added that functionality.

[AliceLR]

The changes here look good to me. As discussed on Discord, these will need to be changed to V293 once the minor version is incremented.