Opacity changes should be recognized

[hmeine]

For better transition animations, it would be necessary to recognize the situations that either

• items (patches) are faded in, e.g. as with latex-beamer’s \setbeamercovered, or that
• items are faded out, e.g. as with
  • latex-beamer’s "overview" slides with a TOC, which transition into the \AtBeginSubsection TOCs where only the current subsection is opaque, or
  • when some content is faded out in order to place text on top of it.