iOS port, expanded SVG filter pipeline, WebKit PSNR test harness

[duncanwilcox]

This consolidates ~55 patches accumulated against IJSVG in Sitely's tree since 2024. The themes:

iOS / iPadOS port

• New IJSVGPlatform.{h,m} centralizes macOS/iOS differences: AppKit/UIKit imports, NSColor/NSImage/NSView/NSFont/NSBezierPath shims, geometry typedefs and macros, an NSValue (IJSVGGeometry) category bridging NS-named methods to their CG-named iOS counterparts, and a UIView (IJSVGNeedsDisplay) category exposing a BOOL-taking -setNeedsDisplay: shim.
• New IJSVGiOSXML.{h,m} provides a libxml2-backed NSXMLNode/Element/Document replacement for iOS (Foundation on iOS doesn't ship NSXML*).
• Existing headers migrated from direct <AppKit/AppKit.h> + <Foundation/Foundation.h> imports to <IJSVG/IJSVGPlatform.h>.

SVG filter pipeline (biggest body of work)

• Filter graph + 7 filter effects on top of the existing IJSVGFilterEffectGaussianBlur: Blend, ColorMatrix, Composite, DisplacementMap, Flood, Lighting, Merge, Offset, Turbulence; plus IJSVGFilterGraph orchestrating the pipeline with CALayer → bitmap replacement rendering.
• filterUnits parsing and filter region attributes honored so SVGs with missing attributes no longer trigger huge-bitmap allocations.
• feOffset rendering fixed; filter export round-trip works.
• Filter rasterization respects the root viewBox transform.
• Filter layer CGImage lifetime made safe across redraws.
• Filter region stabilized at 10% with vImage edge extension.
• Filter-before-opacity rendering: source graphic rendered at full opacity (+26 dB on the gradient-filter baseline).
• Blur calibration matching WebKit (CIGaussianBlur with padded source bitmap, after experimenting with a vImage 3-pass box blur path).
• Turbulence Y-flip corrected.

Gradients / patterns / masks

• spreadMethod="reflect" implemented for linear gradients (+15.8 dB on the car baseline).
• Gradient mask rendering works (masks with gradient content).
• SVG viewport, gradient, and pattern regressions resolved.
• Pattern image transform handling improved.
• Clip-path transform propagation fixed.
• Imported upstream gradient-stop-parsing fixes.

Color space

• All deviceRGB references converted to sRGB throughout the framework.
• rgb() colors use sRGB (+9 dB on the tommek car baseline).

Parser robustness

• Forward-reference prescan so <use> inside <clipPath> can target elements defined later in the document.
• Realloc bug fixed for SVGs with more than 5 transforms.
• NaN/Inf attribute workarounds in gradient parsing.
• IJSVGParser crash on certain SVGs ("circled-arrow-left.svg") fixed.
• IJSVGExporter crashes fixed.

CoreAnimation / memory / stability

• Background-thread CATransaction crash patched.
• Numerous memory leaks fixed.
• @available guards added for APIs not available in 10.14.4 / iOS 17.
• All in-framework logging removed.

Test harness (in IJSVGExample)

• New IJSVGWebKitPSNRTests.m: renders an SVG with IJSVG and snapshots the same SVG in a WKWebView, writes IJSVG/WebKit/diff PNGs and a PSNR results log. Overridable via IJSVG_OUTPUT_DIR and IJSVG_WEBKIT_SETTLE_DELAY env vars.
• IJSVGExampleTests converted to a standalone (logic) test bundle: TEST_HOST/BUNDLE_LOADER removed, IJSVG.framework linked directly and embedded in the .xctest bundle, SVG resources bundled in the test target, scheme promoted from xcuserdata to xcshareddata.
• WebKit.framework linked into the test target.