arc: Set previous point to arc end (#360)

The `arc` command should update the previous point to its end-point.
This might be inconsistent, though, as there are no other commands
setting the previous point to a non user-provided one. Should we add an
option to enable/disable this @fenjalien?

src/draw/shapes.typ

@@ -215,6 +215,8 @@
 /// == Keys
 ///   #show-parameter-block("radius", ("number", "array"), [The radius of the arc. An elliptical arc can be created by passing a tuple of numbers where the first element is the x radius and the second element is the y radius.], default: 1)
 ///   #show-parameter-block("mode", ("string",), [The options are: "OPEN" no additional lines are drawn so just the arc is shown; "CLOSE" a line is drawn from the start to the end of the arc creating a circular segment; "PIE" lines are drawn from the start and end of the arc to the origin creating a circular sector.], default: "OPEN")
+///   #show-parameter-block("update-position", ("bool",), [Update the current canvas position to the arc's end point (anchor `"arc-end"`).
+///     This overrides the default of `true`, that allows chaining of (arc) elements.], default: true)
 ///
 /// = Anchors
 ///   Supports compass anchors when `mode` is "PIE"
@@ -311,7 +313,12 @@
       z
     )
 
-    // center is calculated based on observations of tikz's circular sector and semi circle shapes.
+    // Set the last position to arc-end
+    if style.update-position {
+      ctx.prev.pt = arc-end
+    }
+
+    // Center is calculated based on observations of tikz's circular sector and semi circle shapes.
     let center = if style.mode != "CLOSE" {
       // A circular sector's center anchor is placed half way between the sector-center and arc-center when the angle is 180deg. At 60deg it is placed 1/3 of the way between, this is mirrored at 300deg.
       vector.lerp(

src/styles.typ

@@ -89,6 +89,7 @@
     //   - "PIE"
     mode: "OPEN",
     mark: _default-mark,
+    update-position: true,
   ),
   content: (
     // Allowed values:

tests/arc/last-pos/test.typ

@@ -0,0 +1,23 @@
+#set page(width: auto, height: auto)
+#import "/src/lib.typ": *
+
+#box(stroke: 2pt + red, canvas({
+  import draw: *
+
+  arc((0,0), start: 0deg, stop: 180deg)
+  circle((), radius: .1, fill: blue)
+}))
+
+#box(stroke: 2pt + red, canvas({
+  import draw: *
+
+  arc((0,0), start: 180deg, stop: 0deg)
+  circle((), radius: .1, fill: blue)
+}))
+
+#box(stroke: 2pt + red, canvas({
+  import draw: *
+
+  arc((0,0), start: 180deg, stop: 0deg, update-position: false)
+  circle((), radius: .1, fill: blue)
+}))