0.4.6 - fixed Godot exportet bug, faster Overlay rendering and less resource consumption

This is mostly an emergency update, since I found Godot exporter issue, preventing Tileset to be saved correctly. This is caused by a massive rewrite in 0.4.5 and the absence of autotesting. But there are also a couple of minor UI fixes and performance improvements.

Changes:

- fixed Godot exporter critical bug

- fixed bug with not working preserving of random seed in Overlay mode

- improved performance by: enabling Godot low processor mode, disabling VSync (tile rendering is done by shaders and Godot doesn't have compute ones) and

- fixed a minor UI bug (disabling of random seed input on startup)

- now if input tile part variant has an empty square, that type of tile part is not used - that way you can have some tile parts have random variants and others be the same for every tile