Tests for GUI visual updates

[ivan-mogilko]

Since AGS 3.5.1 there's a number of optimizations done to game GUI, where they don't redraw anytime, but only when something changes in them. Furthermore, depending on a renderer, some properties, such as position may not force redraw of a gui or control itself, since it is done simply by repositioning a texture.

Unfortunately this optimization also turned out to be a source of repeated bugs where GUI did not redraw when it should. Some missing edge cases are found still.

It would be wonderful if game tests could include GUI update tests.

General idea is this:

• put GUI on screen
• start changing its properties
• after each change taken screenshot and test that it was updated.

I am not exactly certain how to "test" though. It may be simply a comparison with the previous saved screenshot to see if anything changed in a region occupied by respective gui or control. Any smarter test would likely require to describe how GUI or control is supposed to look like.