Fix vertical alpha adjustment in wxPixelData<wxImage>::Offset().

Handle "y" parameter correctly when offsetting m_pAlpha, it was just ignored
before, i.e. Offset(x, y) and OffsetY(y) always behaved as if y were 1.

docs/changes.txt

@@ -590,6 +590,7 @@ All (GUI):
 - Fix handling of fast consecutive clicks in wxRibbonBar (atobi).
 - Fix updating nested window scrollbars in some cases (sbrowne).
 - Fix wxPGChoices copy ctor (Snoits).
+- Fix wxPixelData<wxImage>::Offset() for alpha (Markus Rollmann).
 
 wxGTK:
 

include/wx/rawbmp.h

@@ -381,7 +381,7 @@ struct wxPixelDataOut<wxImage>
             {
                 m_pRGB += data.GetRowStride()*y + PixelFormat::SizePixel*x;
                 if ( m_pAlpha )
-                    m_pAlpha += data.GetWidth() + x;
+                    m_pAlpha += data.GetWidth()*y + x;
             }
 
             // move x pixels to the right (again, no row wrapping)
@@ -397,7 +397,7 @@ struct wxPixelDataOut<wxImage>
             {
                 m_pRGB += data.GetRowStride()*y;
                 if ( m_pAlpha )
-                    m_pAlpha += data.GetWidth();
+                    m_pAlpha += data.GetWidth()*y;
             }
 
             // go to the given position