Slow Performance on opening svgs with a DocType like Adobe Ilustrator

[inforithmics]

Description

When there is a svg document with a doctype, the parsing of the svg file is very slow because the doctype is loaded and parsed.
Even if this is unnecessary

Used Versions

UWP https://www.nuget.org/packages/Svg.Skia/0.4.1

Sample svg

for example this svg has a doctype
https://github.com/vvvv/SVG/blob/master/Tests/W3CTestSuite/svg/__issue-318-01.svg?short_path=0c1c253

Workaround

Clean the svgs (removes the doctype and causes the svgs to load faster.
https://github.com/RazrFalcon/svgcleaner-gui

Here I loaded 48 images from which 5 had a doctype and parsing this 5 documents took 2 seconds

[inforithmics]

Pull request for this #783