- fix wxAnimationCtrl::SetBackgroundColour both for generic and native GTK

- fix wxAnimationCtrl::SetBackgroundColour both for generic and native GTK
  version
- fix wxAnimationCtrl::GetAnimation for GTK version (modifying correctly the
  constructor which takes a GdkPixbufAnimation)
- fix the generic wxAnimationCtrl::SetAnimation() when it's used with a
  wxNullAnimation.
- moves the frame counter reset in Stop() as it's more sensed to always have
  m_currentFrame cleared immediately when the animation has been stopped
- fix a problem with transparent bitmaps drawing in wxAnimationCtrl::OnPaint

(part of patch 1598005)

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@43475 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775