Skip to consider Symbol element directly, because it is calculated through reference from use element

Fix #55 : Call Path in a SVG with an empty content element causes an InvalidArgumentException[updated]

Infinite loop in SvgRadialGradientServer CalcScale Issue #163

SvgDocument as Write(XmlTextWriter); allowed elements to decide whether or
not they should be written. E.g. text elements should not be written
unless they have children.

Fixed many attribute enums. Now they will 1) output the proper case, 2) only output if non-null and non-default, 3) include more documentation, 4) are marked as Flags where appropriate.

SvgDocument as Write(XmlTextWriter); allowed elements to decide whether or
not they should be written. E.g. text elements should not be written
unless they have children.

Fixed many attribute enums. Now they will 1) output the proper case, 2) only output if non-null and non-default, 3) include more documentation, 4) are marked as Flags where appropriate.

Stop crash on null or empty images
Deal with multiple spaces in Matrix

More serialization bug fixes and better fallback for font definition
problems.

Added parameter to the SvgAttribute indicating whether the storage is
the _attribute dictionary.  If true, the output will only be serialized
only if the key is in the dictionary.  This structure is an alternate to
the SvgDefinitionDefaults class.

fixed NullReferenceException in TryGetAttribute when _styles dictionary is not set to a valid reference

- Because querying the Bounds property of an ISvgBoundable or
  SvgVisualElement is expensive, we introduced an ImmutableBoundable that
  is used by the SvgRenderer. This class stores the bounds of the supplied
  ISvgBoundable so that multiple queries for the bounds do not hamper
  performance when rendering.

- Converted the Bounds property on ISvgBoundable and SvgVisualElement to a
  CalculateBounds method to indicate that it is an expensive operation
  that returns a new value each time it is called.

- Removed redundant ISvgBoundable Location and Size properties.

- Fixed a bug in SvgFragment.Path property by converting it to a method that
  indicates it returns a new GraphicsPath instance when called and by
  disposing of that instance in CalculateBounds. There are many more
  instances of GraphicsPath not being disposed in the code base but we did
  not address that here.