This file list some things I don't want to forget. There are many possibly
much more important things to do which are not listed here because I won't
forget them.

- Params::Validate consequently
- consequently use in_error where required
- stricter on percentage in colors
- explicit return in all methods
- document engine_class attribute
- introduce engine_args attribute
- documentation on SVG validation.
- method _in_error needs to be public (to be used by State).
- probably, the transformation matrix has to be applied by cairo
  (-> text).
- property validation with dedicated %PROP_VAL?
  -> specific for xsl / css
  -> apply to whole hash after setting of defaults?
  -> element specific?
- callbacks etc. in %ATTR_VAL
- color regexes in Regexes

done
----
- get rid of width/height setting via SVG::Render
- weaken some references (currently not necessary)
- document that 0 width and height are ignored (at least in topmost settings)
- remove white space from attribute values (or not?)
- Acknowledge Cairo library, Perl Cairo module, SVG
- Correct hook docu.
- refer to CHANGES from README
- document which DOM functions have to be provided by an input object
- homogenize parameters for SVG::Rasterize and the rasterize method
- homogenize interface docu of methods
- consequent assignments in interface docu of methods with return value
- regex for engine_class
- svg validation via interface, not class
- revise _create_engine, class name validation, width/height
