[librsvg] (9 commits) ...Merge branch 'johnl/librsvg-even-more-filters'

[Federico Mena Quintero <federico src gnome org>]

Summary of changes:

  bab1596... Implement grayscale filter function
  ed210a2... Replace duplicated num-or-percentage parse functions with h
  637221b... Implement `invert` filter function
  76c1e5e... Implement `brightness()` filter function.
  59dfc6f... Implement `contrast()` filter function.
  cb67d21... Minor cleanup: use .ok() instead of if-let
  d4650c6... Use 'green' rather than 'lime' for the brightness and contr
  4e1ef2a... Whitespace fixups
  1c74312... Merge branch 'johnl/librsvg-even-more-filters'