Fix correct screen DPI and not hard-coded 72 in wxOSX

Don't hard-code screen DPI as 72, use the real number instead (which is now
returned correctly, after the last commit).

Closes #17615.

docs/changes.txt

@@ -120,7 +120,7 @@ wxMSW:
 wxOSX:
 
 - Fix handling of non-BMP characters in GetPartialTextExtents() (ARATA Mizuki).
-- Implement wxGetDisplaySizeMM() correctly (David Vanderson).
+- Implement wxGetDisplaySizeMM() and fix printing DPI (David Vanderson).
 - Remove extra borders around wxFilePickerCtrl (John Roberts).
 - Set up extensions filter correctly in wxFileDialog (nick863).
 - Turn off automatic quotes substitutions in wxTextCtrl (Xlord2).

src/osx/core/printmac.cpp

@@ -585,8 +585,7 @@ bool wxMacPrinter::Print(wxWindow *parent, wxPrintout *printout, bool prompt)
         return false;
     }
 
-    // on the mac we have always pixels as addressing mode with 72 dpi
-    printout->SetPPIScreen(72, 72);
+    printout->SetPPIScreen(wxGetDisplayPPI());
 
     PMResolution res;
     PMPrinter printer;
@@ -764,10 +763,9 @@ void wxMacPrintPreview::DetermineScaling(void)
     int screenWidth , screenHeight ;
     wxDisplaySize( &screenWidth , &screenHeight ) ;
 
-    wxSize ppiScreen( 72 , 72 ) ;
+    wxSize ppiScreen = wxGetDisplayPPI();
     wxSize ppiPrinter( 72 , 72 ) ;
 
-    // Note that with Leopard, screen dpi=72 is no longer a given
     m_previewPrintout->SetPPIScreen( ppiScreen.x , ppiScreen.y ) ;
 
     wxCoord w , h ;