Set minimum size for screenshots and movies

[Keith Hughitt]

In addition to having a maximum allowed screenshot and movie size, there should probably also be some reasonable minimum values. The smallest screen resolutions are about 240x320 and many of the recent smart phones have much higher resolutions. Perhaps something like 120x120 or 160x160 minimum would be good. If the user requests something lower, then the imageScale can be adjusted s.t. the ROI stays the same but the dimensions increase.