Initialize wxGraphicsContext::m_interpolation field.

This member wwas never initialized, resulting in erratic behaviour of
SetInterpolationQuality() which compared its parameter with m_interpolation.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@76297 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/common/graphcmn.cpp

@@ -539,6 +539,7 @@ wxGraphicsContext::wxGraphicsContext(wxGraphicsRenderer* renderer) :
     wxGraphicsObject(renderer),
       m_antialias(wxANTIALIAS_DEFAULT),
       m_composition(wxCOMPOSITION_OVER),
+      m_interpolation(wxINTERPOLATION_DEFAULT),
       m_enableOffset(false)
 {
 }