[librsvg] (13 commits) ...Merge branch 'stacking-context' into 'master'

[Federico Mena Quintero <federico src gnome org>]

Summary of changes:

  b5065a5... with_saved_transform: don't take an Option<Transform>; it i
  b980d3b... Fix misleading comment
  675f7d3... DrawingCtx.with_alpha - no need to be public
  eb46847... with_discrete_layer: take an extra arg with a user-space cl
  075179f... Remove DrawingCtx::with_saved_transform(); pass the transfo
  e5cd4c3... DrawingCtx::draw_from_use_node: don't frob the cr's transfo
  ad7b9bf... New StackingContext struct
  cad4d15... StackingContext: store the opacity here
  63a9113... StackingContext: store the filter property
  ef82864... StackingContext: store the mask
  ec80267... Store the element_name in StackingContext
  6bc3361... acquire_ref the mask node so it doesn't allow cycles
  8a150aa... Merge branch 'stacking-context' into 'master'