Add GetDPIScaleFactor to wxDC and wxGraphicsContext

It can be used to scale coordinates and sizes to make them DPI aware.

In the common headers, use wxDisplay::GetStdPPIValue() as default DPI value,
so the correct values are used on Windows (96) and macOS (72).

In wxMemoryDC use m_contentScaleFactor as the DPIScaleFactor, see e4c2298e5e
(Use window scale factor for all MSW wxDCs associated with windows, 2022-04-04).

include/wx/dc.h

@@ -539,6 +539,8 @@ public:
 
     virtual double GetContentScaleFactor() const { return m_contentScaleFactor; }
 
+    virtual double GetDPIScaleFactor() const { return 1.0; }
+
 #ifdef __WXMSW__
     // Native Windows functions using the underlying HDC don't honour GDI+
     // transformations which may be applied to it. Using this function we can
@@ -827,6 +829,9 @@ public:
     double GetContentScaleFactor() const
         { return m_pimpl->GetContentScaleFactor(); }
 
+    double GetDPIScaleFactor() const
+        { return m_pimpl->GetDPIScaleFactor(); }
+
     // Right-To-Left (RTL) modes
 
     void SetLayoutDirection(wxLayoutDirection dir)

include/wx/dcgraph.h

@@ -101,6 +101,7 @@ public:
     virtual bool CanGetTextExtent() const wxOVERRIDE;
     virtual int GetDepth() const wxOVERRIDE;
     virtual wxSize GetPPI() const wxOVERRIDE;
+    virtual double GetDPIScaleFactor() const wxOVERRIDE;
 
     virtual void SetLogicalFunction(wxRasterOperationMode function) wxOVERRIDE;
 

include/wx/graphics.h

@@ -880,6 +880,7 @@ public:
 
     void SetContentScaleFactor(double contentScaleFactor);
     double GetContentScaleFactor() const { return m_contentScaleFactor; }
+    double GetDPIScaleFactor() const;
 
 #ifdef __WXMSW__
     virtual WXHDC GetNativeHDC() = 0;

include/wx/msw/dc.h

@@ -77,7 +77,7 @@ public:
     virtual bool CanGetTextExtent() const wxOVERRIDE;
     virtual int GetDepth() const wxOVERRIDE;
     virtual wxSize GetPPI() const wxOVERRIDE;
-
+    virtual double GetDPIScaleFactor() const wxOVERRIDE;
 
     virtual void SetMapMode(wxMappingMode mode) wxOVERRIDE;
     virtual void SetUserScale(double x, double y) wxOVERRIDE;

include/wx/msw/dcmemory.h

@@ -27,6 +27,7 @@ public:
     virtual void DoDrawRectangle(wxCoord x, wxCoord y, wxCoord width, wxCoord height) wxOVERRIDE;
     virtual void DoGetSize(int* width, int* height) const wxOVERRIDE;
     virtual void DoSelect(const wxBitmap& bitmap) wxOVERRIDE;
+    virtual double GetDPIScaleFactor() const wxOVERRIDE;
 
     virtual wxBitmap DoGetAsBitmap(const wxRect* subrect) const wxOVERRIDE
     { return subrect == NULL ? GetSelectedBitmap() : GetSelectedBitmap().GetSubBitmapOfHDC(*subrect, GetHDC() );}

src/common/dcgraph.cpp

@@ -24,6 +24,8 @@
     #include "wx/geometry.h"
 #endif
 
+#include "wx/display.h"
+
 //-----------------------------------------------------------------------------
 // Local functions
 //-----------------------------------------------------------------------------
@@ -485,7 +487,12 @@ wxSize wxGCDCImpl::GetPPI() const
 
     // This is the same value that wxGraphicsContext::GetDPI() returns by
     // default.
-    return wxSize(72, 72);
+    return wxDisplay::GetStdPPI();
+}
+
+double wxGCDCImpl::GetDPIScaleFactor() const
+{
+    return m_graphicContext ? m_graphicContext->GetDPIScaleFactor() : 1.0;
 }
 
 int wxGCDCImpl::GetDepth() const

src/common/graphcmn.cpp

@@ -34,6 +34,7 @@
 #endif
 
 #include "wx/private/graphics.h"
+#include "wx/display.h"
 
 //-----------------------------------------------------------------------------
 
@@ -612,6 +613,13 @@ void wxGraphicsContext::SetContentScaleFactor(double contentScaleFactor)
     m_contentScaleFactor = contentScaleFactor;
 }
 
+ double wxGraphicsContext::GetDPIScaleFactor() const
+{
+     wxDouble x, y;
+     GetDPI(&x, &y);
+     return x / (double)wxDisplay::GetStdPPIValue();
+}
+
 #if 0
 void wxGraphicsContext::SetAlpha( wxDouble WXUNUSED(alpha) )
 {
@@ -635,8 +643,8 @@ void wxGraphicsContext::GetDPI( wxDouble* dpiX, wxDouble* dpiY) const
     {
         // Use some standard DPI value, it doesn't make much sense for the
         // contexts not using any pixels anyhow.
-        *dpiX = 72.0;
+        *dpiX = wxDisplay::GetStdPPIValue();
-        *dpiY = 72.0;
+        *dpiY = wxDisplay::GetStdPPIValue();
     }
 }
 
@@ -908,8 +916,8 @@ wxGraphicsContext::CreateLinearGradientBrush(
     return GetRenderer()->CreateLinearGradientBrush
                           (
                             x1, y1,
-                            x2, y2, 
+                            x2, y2,
-                            gradientStops, 
+                            gradientStops,
                             matrix
                           );
 }

src/msw/dc.cpp

@@ -2661,6 +2661,11 @@ wxSize wxMSWDCImpl::GetPPI() const
     return ppi;
 }
 
+double wxMSWDCImpl::GetDPIScaleFactor() const
+{
+    return GetPPI().x / 96.0;
+}
+
 // ----------------------------------------------------------------------------
 // DC caching
 // ----------------------------------------------------------------------------

src/msw/dcmemory.cpp

@@ -143,6 +143,11 @@ void wxMemoryDCImpl::DoSelect( const wxBitmap& bitmap )
     SetFont(GetFont());
 }
 
+double wxMemoryDCImpl::GetDPIScaleFactor() const
+{
+    return m_contentScaleFactor;
+}
+
 void wxMemoryDCImpl::SetFont(const wxFont& font)
 {
     // We need to adjust the font size by the ratio between the scale factor we