[RCORE} Correclty handle window size on resize in auto-scaled HIGHDPI environment

[JeffM2501]

This PR fixes the issue reported in #4834
When we get a resize event from the platform, we are just setting the screen size to the raw data returned by the event.
But if the user enabled the HIGHDPI flag on startup, we scaled the render and window size by the DPI scale. This means that on startup you get one set of sizes for the window, but after a resize, you swap from emulated unscaled values up to scaled values, and GetScreenSize no longer matches the viewport size.

This PR unscales the screen size on resize for the 3 desktop platforms so that the screen size stays in the same 'unscaled' space as the viewport.

[NiamhNightglow]

I just pulled this branch and tested it in my copy of the quickstart, and this does appear to resolve the issues I was seeing with the GetScreenSize functions not returning the scaled values. The values remained scaled when I built my game with this version even with the FLAG_WINDOW_HIGHDPI flag set after rescaling the window.