Stabilize gradient and arc screenshot tests

[shai-almog]

Motivation

• Screenshot tests produced different images across JDK8 and JDK17/21 due to platform-dependent Graphics drawing and coordinate-based arc sizing.
• The intent is to make screenshot output deterministic and independent of JVM rendering differences.

Description

• Replaced on-the-fly Graphics gradient drawing in DrawGradient with a deterministic pixel buffer via a new buildGradientImage method and draw the cached Image instead.
• Added size-aware caching (gradientImage, cachedWidth, cachedHeight) so the pixel buffer is rebuilt only when dimensions change.
• Implemented rasterized gradient helpers (fillLinearGradient, fillRadialGradient, fillRectRadialGradient, blendColor, angleInArc) that produce consistent pixel output using integer RGB buffers and Image.createImage(int[], width, height).
• Fixed FillArc to compute arc width/height from bounds (rather than using coordinate arithmetic) and added a guard to stop when dimensions become non-positive before calling fillArc.

Testing

• No automated tests were executed in this change (mvn/CI runs were not requested).

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Codex Task