jfjoch_test: Bring back TIFFTest with rainbow coloring

tests/TIFFTest.cpp

@@ -58,6 +58,31 @@ TEST_CASE("TIFFTest_File_signed","[TIFF]") {
     REQUIRE_NOTHROW(WriteTIFFToFile("test_image_signed.tiff", image));
 }
 
+rgb rainbowColor(float t) {
+    // Ensure t is in [0,1]
+    t = std::max(0.0f, std::min(1.0f, t));
+
+    // Convert to hue (0 to 6)
+    float hue = t * 6.0f;
+
+    int phase = static_cast<int>(hue);
+    float fract = hue - phase;
+
+    uint8_t p = static_cast<uint8_t>(255 * (1.0f - fract));
+    uint8_t q = static_cast<uint8_t>(255 * fract);
+    uint8_t full = 255;
+
+    switch (phase) {
+        case 0: return {full, q, 0};     // Red to Yellow
+        case 1: return {p, full, 0};     // Yellow to Green
+        case 2: return {0, full, q};     // Green to Cyan
+        case 3: return {0, p, full};     // Cyan to Blue
+        case 4: return {q, 0, full};     // Blue to Magenta
+        case 5: return {full, 0, p};     // Magenta to Red
+        default: return {full, 0, 0};    // Fallback (red)
+    }
+}
+
 TEST_CASE("TIFFTest_File_rgb","[TIFF]") {
     std::vector<rgb> values(512 * 1024);