Fix drawing wxBitmap with both alpha channel and mask

For 32 bpp wxBitmap with both alpha channel and mask we have to apply mask on our own while drawing the bitmap because MaskBlt() API doesn't work properly with 32 bpp RGBA bitmaps. To do so we need to create a temporary bitmap with copy of original RGB data and with alpha channel being a superposition of the original alpha values and the mask.

See #18498.

src/msw/bitmap.cpp

@@ -1259,6 +1259,53 @@ void wxBitmap::MSWUpdateAlpha()
 #endif // wxUSE_WXDIB/!wxUSE_WXDIB
 }
 
+void wxBitmap::MSWBlendMaskWithAlpha()
+{
+#if defined(wxHAS_RAW_BITMAP)
+    if ( !HasAlpha() || !GetMask() )
+        return;
+    // Update alpha channel by blending original alpha values with mask and then remove the mask.
+    // For non-masked pixels alpha channel values will remain intact and for masked pixels
+    // they will be set to the transparent value (0).
+    AllocExclusive();
+
+    {
+        wxBitmap bmpMask = GetMask()->GetBitmap();
+        wxNativePixelData maskData(bmpMask);
+        wxCHECK_RET(maskData, "No access to bitmap mask data");
+
+        wxAlphaPixelData bmpData(*this);
+        wxCHECK_RET(bmpData, "No access to bitmap data");
+
+        const int w = GetWidth();
+        const int h = GetHeight();
+
+        wxNativePixelData::Iterator maskRowStart(maskData);
+        wxAlphaPixelData::Iterator bmpRowStart(bmpData);
+        for ( int y = 0; y < h; y++ )
+        {
+            wxNativePixelData::Iterator pMask = maskRowStart;
+            wxAlphaPixelData::Iterator pBmp = bmpRowStart;
+            for ( int x = 0; x < w; x++, ++pBmp, ++pMask )
+            {
+                // Masked pixel is not drawn i.e. is transparent,
+                // non-masked pixel is untouched.
+                if ( pMask.Red() == 0 )
+                {
+                    pBmp.Red() = pBmp.Green() = pBmp.Blue() = 0; // pre-multiplied
+                    pBmp.Alpha() = wxALPHA_TRANSPARENT;
+                }
+            }
+            maskRowStart.OffsetY(maskData, 1);
+            bmpRowStart.OffsetY(bmpData, 1);
+        }
+    }
+    GetBitmapData()->SetMask((wxMask*)NULL);
+    wxASSERT(HasAlpha());
+    wxASSERT(!GetMask());
+#endif // wxHAS_RAW_BITMAP
+}
+
 // ----------------------------------------------------------------------------
 // wxBitmap setters
 // ----------------------------------------------------------------------------