Use more accurate DwmGetWindowAttribute for getting window bounds on Windows

[zcanann]

GetWindowRect() includes window drop shadows, which can cause attempts to screen capture outside of window boundaries, resulting in black bars

DwmGetWindowAttribute is more accurate, including the window frame, but nothing outside the window bounds.

[Brendonovich]

DwmGetWindowAttribute does seem to be more accurate but apparently doesn't include DPI scaling like GetWindowRect, we'll have to make them equivalent before using this different API.