Store alpha channel values when converting internal D2D bitmap to wxImage.

Save alpha channel retrieved from internal bitmap if target wxImage is capable to store it. Also undo premultiplication of RGB values if bitmap contains alpha channel values.

Closes #17311.

src/msw/graphicsd2d.cpp

@@ -2293,14 +2293,28 @@ private:
         BYTE* buffer = new BYTE[bufferSize];
         m_wicBitmap->CopyPixels(NULL, 4 * width, bufferSize, buffer);
         unsigned char* dest = m_resultImage->GetData();
+        unsigned char* destAlpha = m_resultImage->GetAlpha();
+
+        WICPixelFormatGUID pixelFormat;
+        m_wicBitmap->GetPixelFormat(&pixelFormat);
+        wxASSERT_MSG( pixelFormat == GUID_WICPixelFormat32bppPBGRA ||
+                  pixelFormat == GUID_WICPixelFormat32bppBGR,
+                  wxS("Unsupported pixel format") );
+
+        // Only premultiplied ARGB bitmaps are supported.
+        const bool hasAlpha = pixelFormat == GUID_WICPixelFormat32bppPBGRA;
 
         int k = 0;
         while (k < width * height)
         {
             wxPBGRAColor color = wxPBGRAColor(buffer + k * 4);
-            dest[k * 3 + 0] = color.r;
-            dest[k * 3 + 1] = color.g;
-            dest[k * 3 + 2] = color.b;
+            unsigned char a =  hasAlpha ? color.a : 255;
+            // Undo premultiplication for ARGB bitmap
+            dest[k * 3 + 0] = (a > 0 && a < 255) ? ( color.r * 255 ) / a : color.r;
+            dest[k * 3 + 1] = (a > 0 && a < 255) ? ( color.g * 255 ) / a : color.g;
+            dest[k * 3 + 2] = (a > 0 && a < 255) ? ( color.b * 255 ) / a : color.b;
+            if ( destAlpha )
+                *destAlpha++ = a;
             ++k;
         }