w32_common: use the screen working area when resizing the window

This makes the default fit on screen, autofit and window-scale
changing behavior to use the screen working area, instead of
the whole screen area.

As a result mpv window doesn't cover the taskbar now when opening
videos with size larger than the screen size.

The actual behavior now is the same as expected behavior for
usecases 1-4 from #4363.

This commit also removes the screenrc from w32 struct.
The screen rect can now be retrieved via `get_screen_area` function,
which was renamed from `update_screen_rect`.

On a multi-monitor system, if the user moved the window between
monitors, this function will return the current screen area under
the window, and not the screen area from monitor specified by
`--screen` option. The `--screen` option sets the initial monitor
the mpv window is displayed on.

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