Win32: support arbitrary DPI awareness contexts #307

Currently, the Win32 platform always sets the newest DPI awareness for the windows it creates.

However, child windows cannot have a different DPI awareness than its parents, as highlighted in this part of the Windows documentation:

Forced reset of process-wide DPI awareness

In general, the DPI awareness mode of your process cannot be changed after process initialization. Windows can, however, forcibly change the DPI awareness mode of your process if you attempt to break the requirement that all HWNDs in a window tree have the same DPI awareness mode. On all versions of Windows, as of Windows 10 1703, it is not possible to have different HWNDs in an HWND tree run in different DPI awareness modes. If you attempt to create a child-parent relationship that breaks this rule, the DPI awareness of the entire process can be reset. This can be triggered by:

• A CreateWindow call where the passed in parent window is of a different DPI awareness mode than the calling thread.
• A SetParent call where the two windows are associated with different DPI awareness modes.

So in the parented case, baseview should query the DPI awareness of the current window, and only use this one throughout various DPI-aware operations.