Add ability to fix VGA window size

[ian-rose]

There was an issue about adding scaling to the VGA window (#800), and in the resolution of that issue the ability to re-size the VGA window was added. This is great, but doesn't actually work very well when trying to identify issues with generating specific pixels, as the scaling is not always a 1:1 mapping (for instance, scaling 640 pixels to a random size of 800 makes different pixels display at different widths). It would be great to allow for both the current dynamic resizing, as well as setting a fixed size (1:1, 2:1, etc...) so all pixels are represented identically. Given many operating systems round the corners of windows it would be nice to be able to size the window larger than the VGA output so the entirety of the output is displayed within the rounded corners of the parent window.