High DPI support

zgt.Window contains a source_dpi field which by defaukt is set to 96. The field sets for what DPI the app was developed against. For example, the default value means that every pixel is 1/96th of an inch. Images are also assumed to be in source_dpi, unless they contain metadata indicating a different DPI

If you're using this system with scalable icons (like TVG), this system allows seamless integration with High-DPI displays, with fractional scaling and support for multi-monitor setups with different DPIs