Win32: Better DPI changes handling

[prokopyl]

This PR improves the Win32 DPI changes handling in several ways:

• Separate storage of current_size and current_scale_factor, allowing us to track which one changed;
  • This supersedes the fix in Fix WM_DPICHANGED not triggering Resized event #140.
• Store the above information in Cells instead of RefCell;
  • This is simpler and fixes Windows: Panic "already borrowed" when reentering wnd_proc due to mouse move or timer #143.
• Adjust WM_DPICHANGED event handler to properly use the RECT given with the event;
  • This fixes the window positioning itself weirdly when moving it between display.
• Properly trigger a Resize event if the above results in changing the size;
  • It turns out the WM_DPICHANGED often happens after the WM_SIZE event when changing the DPI of the window, leaving the event handler "behind" until the window was resized again.
• Fix how the scaling factor was read and initialized on initial window creation.
  • This fixes the initial frame being only partially covering the window on a non-96dpi display.

This is a leaner version of #178, leaving the more "refactor-y" parts of it for later.
Note that there are still some DPI-related issues even after this PR, but they require some larger refactoring and usage of DPI-aware Win32 functions (e.g. AdjustWindowRectExForDpi, SetThreadDpiAwarenessContext, etc.), so we'll do these after the v0.1 release.