Fix bug with clearing shared alpha channel in wxImage::ClearAlpha().

Allocate our own exclusive copy of the data before modifying it, otherwise
calling ClearAlpha() would clear alpha not only for the image it was called on
but also for all the other images sharing data with it.

docs/changes.txt

@@ -591,6 +591,7 @@ All (GUI):
 - Fix updating nested window scrollbars in some cases (sbrowne).
 - Fix wxPGChoices copy ctor (Snoits).
 - Fix wxPixelData<wxImage>::Offset() for alpha (Markus Rollmann).
+- Fix bug in wxImage::ClearAlpha() for shared data (Markus Rollmann).
 
 wxGTK:
 

src/common/image.cpp

@@ -2040,6 +2040,8 @@ void wxImage::ClearAlpha()
 {
     wxCHECK_RET( HasAlpha(), wxT("image already doesn't have an alpha channel") );
 
+    AllocExclusive();
+
     if ( !M_IMGDATA->m_staticAlpha )
         free( M_IMGDATA->m_alpha );