Revert "Optimize drawing of small bitmaps with alpha in wxMSW"

This reverts commit d04e256993 because it
results in crashes due to writing out of bounds of the bitmap: nothing
guarantees that the entire (x, y, dstWidth, dstHeight) rectangle fits in the
destination bitmap and so the code of this commit could merrily overflow it
and did it as could be seen e.g. in the HTML test sample after scrolling
around a little.

See #16766.

src/msw/dc.cpp

@@ -2624,12 +2624,11 @@ static bool AlphaBlt(wxMSWDCImpl* dcDst,
             if ( data )
             {
                 wxAlphaPixelData::Iterator p(data);
-                p.Offset(data, x, y);
-                for ( int yDst = 0; yDst < dstHeight; yDst++ )
+                for ( int old_y = 0; old_y < data.GetHeight(); old_y++ )
                 {
                     wxAlphaPixelData::Iterator rowStart = p;
 
-                    for ( int xDst = 0; xDst < dstWidth; xDst++ )
+                    for ( int old_x = 0; old_x < data.GetWidth(); old_x++ )
                     {
                         // We choose to use wxALPHA_TRANSPARENT instead
                         // of perhaps more logical wxALPHA_OPAQUE here