FlatSVGIcon: some additions to PR #817:

- create new `LinearGradientPaint` only if color has changed
- preserve old `LinearGradientPaint.colorSpace` and `transform`

flatlaf-testing/src/main/resources/com/formdev/flatlaf/testing/extras/jsvg/LICENSE.txt

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+The SVG icons in this folder are from JSVG,
+which is licensed under the MIT License
+See: https://github.com/weisJ/jsvg

flatlaf-testing/src/main/resources/com/formdev/flatlaf/testing/extras/jsvg/gradientText0.svg

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+<svg viewBox="0 0 100 100" xmlns="http://www.w3.org/2000/svg"
+	stroke="none">
+	<defs>
+		<linearGradient id="myGradient"
+			gradientTransform="rotate(90)">
+			<stop offset="5%" stop-color="gold" />
+			<stop offset="95%" stop-color="red" />
+		</linearGradient>
+	</defs>
+	<rect x="0" y="50" width="100" height="40" fill="url(#myGradient)" />
+	<text x="0" y="50" fill="url(#myGradient)">Hello lovely World!</text>
+</svg>

flatlaf-testing/src/main/resources/com/formdev/flatlaf/testing/extras/jsvg/gradientText1.svg

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+<svg viewBox="0 0 100 100" xmlns="http://www.w3.org/2000/svg"
+	stroke="none">
+	<defs>
+		<linearGradient id="myGradient">
+			<stop offset="5%" stop-color="gold" />
+			<stop offset="95%" stop-color="red" />
+		</linearGradient>
+	</defs>
+	<text x="0" y="50">
+		Hello
+		<tspan fill="url(#myGradient)">lovely</tspan>
+		World!
+	</text>
+</svg>

flatlaf-testing/src/main/resources/com/formdev/flatlaf/testing/extras/jsvg/gradientText2.svg

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+<svg viewBox="0 0 100 100" xmlns="http://www.w3.org/2000/svg"
+	stroke="none">
+	<defs>
+		<linearGradient id="myGradient">
+			<stop offset="5%" stop-color="gold" />
+			<stop offset="95%" stop-color="red" />
+		</linearGradient>
+	</defs>
+	<text x="0" y="50" fill="url(#myGradient)">
+		Hello
+		<tspan>lovely</tspan>
+		World!
+	</text>
+</svg>

flatlaf-testing/src/main/resources/com/formdev/flatlaf/testing/extras/jsvg/linearGradient.svg

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+<svg width="120" height="240" version="1.1"
+	xmlns="http://www.w3.org/2000/svg">
+	<defs>
+		<linearGradient id="Gradient1"
+			gradientUnits="objectBoundingBox">
+			<stop offset="0%" stop-color="red" />
+			<stop offset="50%" stop-color="black" stop-opacity="0" />
+			<stop offset="100%" stop-color="blue" />
+		</linearGradient>
+		<linearGradient id="Gradient2" x1="0" x2="0" y1="0"
+			y2="1">
+			<stop offset="0%" stop-color="red" />
+			<stop offset="50%" stop-color="black" stop-opacity="0" />
+			<stop offset="100%" stop-color="blue" />
+		</linearGradient>
+	</defs>
+
+	<rect id="rect1" x="10" y="10" rx="15" ry="15" width="100"
+		height="100" fill="url(#Gradient1)" />
+	<rect x="10" y="120" rx="15" ry="15" width="100" height="100"
+		fill="url(#Gradient2)" />
+
+</svg>

flatlaf-testing/src/main/resources/com/formdev/flatlaf/testing/extras/jsvg/stripes.svg

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+<svg xmlns="http://www.w3.org/2000/svg" width="500px" height="500px">
+	<defs>
+		<linearGradient id="stripes">
+			<stop offset="0%" stop-color="red" />
+			<stop offset="10%" stop-color="red" />
+			<stop offset="10%" stop-color="orange" />
+			<stop offset="20%" stop-color="orange" />
+			<stop offset="20%" stop-color="yellow" />
+			<stop offset="30%" stop-color="yellow" />
+			<stop offset="30%" stop-color="lightgreen" />
+			<stop offset="40%" stop-color="lightgreen" />
+			<stop offset="40%" stop-color="green" />
+			<stop offset="50%" stop-color="green" />
+			<stop offset="50%" stop-color="purple" />
+			<stop offset="60%" stop-color="purple" />
+			<stop offset="60%" stop-color="blue" />
+			<stop offset="70%" stop-color="blue" />
+			<stop offset="70%" stop-color="skyblue" />
+			<stop offset="80%" stop-color="skyblue" />
+			<stop offset="80%" stop-color="pink" />
+			<stop offset="90%" stop-color="pink" />
+			<stop offset="90%" stop-color="white" />
+			<stop offset="100%" stop-color="white" />
+		</linearGradient>
+	</defs>
+	<rect width="100%" height="100%" fill="url(#stripes)" />
+</svg>