Add system option to never use AutoSystemDpiAware

Add the option 'msw.native-dialogs-pmdpi' that will always show the native msw dialogs
as per monitor dpi aware, even when there are displays with different dpi.

include/wx/msw/private/dpiaware.h

@@ -16,6 +16,7 @@
 
 #include "wx/dynlib.h"
 #include "wx/display.h"
+#include "wx/sysopt.h"
 
 namespace wxMSWImpl
 {
@@ -68,7 +69,12 @@ public:
 
     static bool Needed()
     {
-        // use system-dpi-aware context when there are displays with different DPI
+        // use system-dpi-aware context when:
+        // - the user did not set an option to force per-monitor context
+        // - there are displays with different DPI
+        if ( wxSystemOptions::GetOptionInt("msw.native-dialogs-pmdpi") == 1 )
+            return false;
+
         bool diferentDPI = false;
         for ( unsigned i = 1; i < wxDisplay::GetCount() && !diferentDPI; ++i )
             diferentDPI = wxDisplay(0u).GetPPI() != wxDisplay(i).GetPPI();

interface/wx/sysopt.h

@@ -81,6 +81,12 @@
         using it, i.e. this has the same effect as calling
         wxApp::MSWEnableDarkMode(). If set to 2, use dark mode unconditionally,
         as if this function were called with wxApp::DarkMode_Always argument.
+    @flag{msw.native-dialogs-pmdpi}
+        Some native win32 dialogs (like the font and colour pickers) are not
+        per-monitor DPI aware, and wxWidgets will forcefully show them as
+        system DPI aware when there are monitors with different DPI connected.
+        If set to 1, these dialogs will always be shown as per-monitor DPI
+        aware (when enabled in the manifest).
     @endFlagTable