win32: Fix edge-case bug with DPI scaling

[grokys]

What does the pull request do?

Fixes an issue reported to us by @danipen that can be reproduced by https://github.com/danipen/AvaloniaDpiIssue:

• Have two monitors, the primary monitor with 125% scaling the secondary with 100% scaling
• Run the repro
• Move the window to the secondary monitor
• Show the dialog
• See that the dialog is cropped

This appears to be caused by a win32 bug. Not sure I can describe it any better than the in-code comment:

// I can't find mention of this *anywhere* online, but it seems that setting
// GWL_HWNDPARENT to a window which is on the non-primary monitor can cause two
// WM_DPICHANGED messages to be sent: the first changing the DPI to the parent's DPI,
// then another changing the DPI back. This then causes Windows to provide an incorrect
// suggested new rectangle to the WM_DPICHANGED message if the window is immediately
// moved to the parent window's monitor (e.g. when using
// WindowStartupLocation.CenterOwner) causing the window to be shown with an incorrect
// size.
//
// Just ignore any WM_DPICHANGED while we're setting the parent as this shouldn't
// change the DPI anyway.

The solution seems to be to simply ignore DPI changes while GWL_HWNDPARENT is being set.

[maxkatz6]

Looks weird, but let's see if it regresses on master with anything