Added local overlay tiles, rendering even if overlay is unavailable

[amamic1803]

Hi,

I've added support for local images as overlay tiles. I also edited that part of code to show map even if overlay tile isn't available for that location. Finally, I added 2 lines to example in README.md to show how local overlays can be used.

I've tested and everything appears to work correctly as I wanted. Don't think I broke anything.

Regards,
Antonio Mamić

[amamic1803]

I fixed zooming with mouse wheel on windows. Before, it would sometimes skip a zoom level, now when scrolling, it must go through all zoom levels. (I only did this in the part of code meant for windows because I don't know how it works on other systems).

[amamic1803]

When changing window/widget size, minimum zoom level must be increased sometimes to avoid not having enough tiles to fill the screen. It was set to only check width, but I fixed it so it also checks height now as it should. Also when changing window/widget size, even if zoom change isn't needed, map must be redrawn to place top left tile in top left corner, otherwise it would stay at old location and empty tiles would be shown. I fixed that also.