Fix crash when creating wxGraphicsBitmap from cairo_surface_t.

Initialize m_buffer to avoid crashes when deleting it in Cairo-based
wxGraphicsContext implementation for bitmaps created from the native ones.

Don't leave the bitmap size uninitialized neither, this might not result in
crashes, but is definitely wrong as well.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@77718 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Document wxGraphicsContext::CreateBitmapFromNativeBitmap() ownership rules.

Mention that it takes ownership of the native bitmap passed to it.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@77719 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775