added support for image resolution options to PNG handler (heavily modified patch 1704128)

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@45567 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/image.h

@@ -37,9 +37,17 @@
 #define wxIMAGE_OPTION_RESOLUTIONUNIT        wxString(_T("ResolutionUnit"))
 
 // constants used with wxIMAGE_OPTION_RESOLUTIONUNIT
-enum
+//
+// NB: don't change these values, they correspond to libjpeg constants
+enum wxImageResolution
 {
+    // Resolution not specified
+    wxIMAGE_RESOLUTION_NONE = 0,
+
+    // Resolution specified in inches
     wxIMAGE_RESOLUTION_INCHES = 1,
+
+    // Resolution specified in centimeters
     wxIMAGE_RESOLUTION_CM = 2
 };
 
@@ -112,6 +120,13 @@ protected:
     bool CallDoCanRead(wxInputStream& stream);
 #endif // wxUSE_STREAMS
 
+    // helper for the derived classes SaveFile() implementations: returns the
+    // values of x- and y-resolution options specified as the image options if
+    // any
+    static wxImageResolution
+    GetResolutionFromOptions(const wxImage& image, int *x, int *y);
+
+
     wxString  m_name;
     wxString  m_extension;
     wxString  m_mime;