Add method to convert wxDIB to wxImage enforcing alpha channel values (wxMSW).

wxDIB::ConvertToImage called with Convert_AlphaAuto converts wxDIB to wxImage with automatic checking if 32 bpp DIB contains real alpha values (legacy way).
When it is called with Convert_AlphaAlwaysIf32bpp then automatic checking is disabled and 32 bpp DIB is unconditionally converted to ARGB wxImage.

include/wx/msw/dib.h

@@ -164,7 +164,18 @@ public:
     bool Create(const wxImage& image, PixelFormat pf = PixelFormat_PreMultiplied);
 
     // create wxImage having the same data as this DIB
-    wxImage ConvertToImage() const;
+
+    // Possible options of conversion to wxImage
+    enum ConversionFlags
+    {
+        // Determine whether 32bpp DIB contains real alpha channel
+        // and return wxImage with or without alpha channel values.
+        Convert_AlphaAuto,
+        // Assume that 32bpp DIB contains valid alpha channel and always
+        // return wxImage with alpha channel values in this case.
+        Convert_AlphaAlwaysIf32bpp
+    };
+    wxImage ConvertToImage(ConversionFlags flags = Convert_AlphaAuto) const;
 #endif // wxUSE_IMAGE
 
 

src/msw/dib.cpp

@@ -676,7 +676,7 @@ bool wxDIB::Create(const wxImage& image, PixelFormat pf)
     return true;
 }
 
-wxImage wxDIB::ConvertToImage() const
+wxImage wxDIB::ConvertToImage(ConversionFlags flags) const
 {
     wxCHECK_MSG( IsOk(), wxNullImage,
                     wxT("can't convert invalid DIB to wxImage") );
@@ -776,7 +776,7 @@ wxImage wxDIB::ConvertToImage() const
     if ( hasOpaque && hasTransparent )
         hasAlpha = true;
 
-    if ( !hasAlpha && image.HasAlpha() )
+    if ( !hasAlpha && image.HasAlpha() && flags == Convert_AlphaAuto )
         image.ClearAlpha();
 
     return image;