Fix retrieving wxMask from wxBitmap in wxMSW

In wxMask::GetBitmap() we need to create a copy of the mask bitmap and assign
it to the resulting wxBitmap. We cannot simply assign existing mask bitmap to
the resulting wxBitmap because when wxMask::GetBitmap would be called more
then once then many independent wxBitmaps would own the same bitmap.

Closes https://github.com/wxWidgets/wxWidgets/pull/230

Closes #17395.

src/msw/bitmap.cpp

@@ -1574,8 +1574,41 @@ bool wxMask::Create(const wxBitmap& bitmap, const wxColour& colour)
 
 wxBitmap wxMask::GetBitmap() const
 {
+    // We have to do a deep copy of the mask bitmap
+    // and assign it to the resulting wxBitmap.
+
+    // Create new bitmap with the same parameters as a mask bitmap.
+    BITMAP bm;
+    ::GetObject(m_maskBitmap, sizeof(bm), (LPVOID)&bm);
+
+    HBITMAP hNewBitmap = ::CreateBitmapIndirect(&bm);
+    if ( !hNewBitmap )
+    {
+        wxLogLastError(wxS("CreateBitmapIndirect"));
+        return wxNullBitmap;
+    }
+
+    // Copy the bitmap.
+    HDC hdcSrc = ::CreateCompatibleDC((HDC)NULL);
+    HBITMAP hSrcOldBmp = (HBITMAP)::SelectObject(hdcSrc, m_maskBitmap);
+
+    HDC hdcMem = ::CreateCompatibleDC((HDC)NULL);
+    HBITMAP hMemOldBmp = (HBITMAP)::SelectObject(hdcMem, hNewBitmap);
+
+    ::BitBlt(hdcMem, 0, 0, bm.bmWidth, bm.bmHeight, hdcSrc, 0, 0, SRCCOPY);
+    ::SelectObject(hdcMem, hMemOldBmp);
+
+    // Clean up.
+    ::SelectObject(hdcSrc, hSrcOldBmp);
+    ::DeleteDC(hdcSrc);
+    ::DeleteDC(hdcMem);
+
+    // Create and return a new wxBitmap.
     wxBitmap bmp;
-    bmp.SetHBITMAP(m_maskBitmap);
+    bmp.SetHBITMAP((WXHBITMAP)hNewBitmap);
+    bmp.SetSize(bm.bmWidth, bm.bmHeight);
+    bmp.SetDepth(bm.bmPlanes);
+
     return bmp;
 }