[orx-fcurve] improve handling of absolute hold command

orx-fcurve/src/commonMain/kotlin/FCurve.kt

@@ -308,11 +308,8 @@ class FCurveBuilder {
         if (relative) {
             lineTo(x, cursor.y)
         } else {
-            val d = x - cursor.x
-            require(d >= 0.0) {
-                "requested to hold until $x, but cursor is already at ${cursor.x}"
-            }
-            lineTo(d, cursor.y)
+            require(segments.isEmpty()) { "absolute hold (H $x) is only allowed when used as first command"}
+            cursor = cursor.copy(x = x)
         }
         path += "h$x"
     }

orx-fcurve/src/commonTest/kotlin/TestFCurve.kt

@@ -16,4 +16,20 @@ class TestFCurve {
         assertEquals(10.5, normalizedSampler(1.0))
         assertEquals(10.5, normalizedSampler(-1.0))
     }
+
+    @Test
+    fun testAbsoluteHold() {
+        run {
+            val text = "H-1 L 5 5"
+            val fc = fcurve(text)
+            assertEquals(0.0, fc.value(-1.0))
+            assertEquals(5.0, fc.value(4.0))
+        }
+        run {
+            val text = "H1 L 5 5"
+            val fc = fcurve(text)
+            assertEquals(0.0, fc.value(1.0))
+            assertEquals(5.0, fc.value(6.0))
+        }
+    }
 }