reference: Add lerpColor

Signed-off-by: Ce Gao <ce.gao@outlook.com>

examples/reference/lerpColor/lerpColor1/.test.yml

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+test:
+  reference: https://processing.org/reference/images/lerpColor_.png

examples/reference/lerpColor/lerpColor1/lerpColor1.rpde

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+stroke(255)
+background(51)
+from = color(204, 102, 0)
+to = color(0, 102, 153)
+interA = lerpColor(from, to, 0.33)
+interB = lerpColor(from, to, 0.66)
+fill(from)
+rect(10, 20, 20, 60)
+fill(interA)
+rect(30, 20, 20, 60)
+fill(interB)
+rect(50, 20, 20, 60)
+fill(to)
+rect(70, 20, 20, 60)

src/test/e2e/core/function/LerpColor1Test.java

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+
+// Generated by hack/generate-e2e-test.py
+package test.e2e.core.function;
+
+import static org.junit.Assert.fail;
+import org.junit.Test;
+import test.e2e.core.E2eTestBase;
+
+public class LerpColor1Test extends E2eTestBase {
+
+  public LerpColor1Test() {
+    coreCodeTemplate = "stroke(255)\nbackground(51)\nfrom = color(204, 102, 0)\nto = color(0, 102, 153)\ninterA = lerpColor(from, to, 0.33)\ninterB = lerpColor(from, to, 0.66)\nfill(from)\nrect(10, 20, 20, 60)\nfill(interA)\nrect(30, 20, 20, 60)\nfill(interB)\nrect(50, 20, 20, 60)\nfill(to)\nrect(70, 20, 20, 60)\n\nsaveFrame(\"%s\")\nexit()\n";
+    referenceURI = "https://processing.org/reference/images/lerpColor_.png";
+  }
+
+  @Test
+  public void test() {
+    try {
+      defaultOperation();
+    } catch (Exception exception) {
+      System.err.println(exception);
+      fail("Should not have thrown any exception");
+    }
+  }
+}