add XML header and support for disabling the BOM in SvgDocument.cs

1. Added XML Header, this is conform according to https://www.w3.org/TR/SVG/struct.html
2. Added support for removing Byte Order Mark (BOM), this needed in some cases, eg. if the SVG is used as SSI (Server Side Include) in IIS (Internet Information Server).

Revert "Fix #176: Blurry text when overlapped with colored background"

Proper SvgTextBase.Text property setter behaviour

SvgTextBase Text property setter resets Node collection to have single content node with provided text.

Circle, ellipse, polygon and rectangle render correctly when their stroke-width is set to zero

These shapes now render their fill, even if StrokeWidth less or equals zero.

RenderStroke method won't render the stroke if StrokeWidth is less or equal zero.

Copy the nodes of a SvgElement when creating a deep copy.

Nested svg areas are misplaced or invisible 

SvgPointCollection ToString method fix

Fix fill-opacity/stroke-opacity wrongly inheriting from opacity when …

Fix stack overflow exception for em font-size value type

Added a test with a SvgText element where the whole text element was not written to XML of a deep copy of the SvgDocument.

If the root svg uses transformations ( e.g. defines a viewBox and
different width and height) the nested svg areas are misplaced or
invisible.

SvgPointCollection.ToString method modified to return valid coordinate string with each coordinate pair separated by whitespace and their x and y values separated by comma.

made SvgImage.GetImage public

fixed SvgImage.GetImage

[bugfix] now pattern reads proper transform named "patternTransform"

changed rounding behavior of float to int color conversion from floor() to round()

Issue 225 - Image with large embedded image fails to render

reference: https://developer.mozilla.org/en-US/docs/Web/SVG/Element/pattern

This is huge especially for the low values because human eyes distinguish darker colors much better.

- Changed SvgImage.Href property from Uri type to String to avoid UriFormatException if Uri contains too much data
- Changed GetImage to receive a string instead of Uri object.
  - Created a temp Uri object in method with substring of allowed length and using this to determine Uri characteristics