Fixed bitmap size calculation in SvgDocument.Draw(); Added guard on case when exception occured during rendering.

- SvgGradientStop offset values were not being confined to percentages (0.0-1.0 or %) so a ToPercentage() method has been added to SvgUnit which converts to a percentage unit. This method is called during the Offset property setter.
- SvgLinearGradientServer was incorrectly calculating the end point of the gradient.

- Added initial custom/querystring 'parameters' implementation (#5652)

- Fixed support for text-anchor="middle" and text-anchor="end" for <text/> elements.
- Added more XML API documentation
- Added x attribute and y attribute support to <use/> elements.