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/trunk@76299 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Fix mismatch between GDI+ and wxGraphicsContext interpolation quality.

Use wxGDIPlusContext own method instead of using GDI+ method directly when
setting the interpolation mode in wxGDIPlusContext initialization code to
ensure that m_interpolation field matches the real interpolation mode used.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76300 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Remove "default" clause from enums in wxGDIPlusContext code.

This will allow the compiler to give us a warning if more elements are added
to the enum later but not added to the switch.

Also add error checking to Graphics::SetSmoothingMode() call.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76301 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775