[vector_graphics_compiler] support percentage units SVG shape attributes

[stevenctl]

Adds percentage unit support for SVG shape attributes

SVG shapes like <rect>, <circle>, <ellipse>, and <line> can have percentage values for their position and size attributes (e.g., width="50%", cx="50%"). Previously these weren't handled correctly. Easy to test with any image like https://placeholdit.com/600x400/dddddd/999999

This adds:

• Percentage parsing in parseDoubleWithUnits that resolves values against the viewport dimensions
• Viewport width/height getters on SvgParser
• Proper percentage reference handling for all basic shape elements
• Circle radius uses the normalized diagonal per SVG spec

List which issues are fixed by this PR. You must list at least one issue.

I'm not sure if it fixes these issues, as they have no description. I can file a new issue if needed.

https://github.com/flutter/flutter/issues/158844 https://github.com/flutter/flutter/issues/158845

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