[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.

flutter/flutter#158844
flutter/flutter#158845

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[gemini-code-assist]

Code Review

This pull request adds support for percentage units in SVG shape attributes like rect, circle, ellipse, and line, which is a valuable feature. The changes correctly resolve percentage values against the viewport dimensions, and the circle radius calculation correctly uses the normalized diagonal as per the SVG specification. The addition of new tests to verify this functionality is also great. I've found a minor issue with a misleading comment in the new percentage parsing logic and have suggested a correction to improve clarity. Overall, the changes are well-implemented.

[gemini-code-assist]

The comment here is a bit misleading. The code doesn't treat unresolved percentages as 0; it either returns null or throws a FormatException. Also, this is a change in behavior from before, where 50% would be parsed as 50.0, so the claim of maintaining backwards compatibility isn't quite accurate (though the new behavior is more correct). I suggest updating the comment to more accurately reflect the code's logic and the SVG specification's requirements.

Suggested change

	// If no reference dimension is available, treat as 0.
	// This maintains backwards compatibility for cases where
	// percentages can't be resolved.
	// If no reference dimension is available, the percentage cannot be resolved.
	// Per the SVG specification, this is an error.