This PR adds a semicircle primitive.

This draws circular arcs.

The control point position allows the user to change segment into
a line or flipped arc.

@@ -1635,6 +1636,7 @@ def brush_basic_gpencil_paint_settings(layout, context, brush, *, compact=False)
             "builtin.box",
             "builtin.circle",
             "builtin.polyline",
+            "builtin.semicircle",

@@ -135,1 +138,4 @@
 
+  float2 start_drag_position_2d;
+  float2 end_drag_position_2d;
+  bool reverse;

@@ -136,0 +139,4 @@
+  float2 start_drag_position_2d;
+  float2 end_drag_position_2d;
+  bool reverse;
+  bool flip;

@@ -137,2 +145,4 @@
 };
 
+/* Copied directly from v3 private function in mesh_fair.cc but using float 2 parameters. */
+static float2 calc_circumcenter_2d(const float2 v1, const float2 v2, const float2 v3)

@@ -308,0 +439,4 @@
+        }
+        else {
+          const float2 center_pos = calc_circumcenter_2d(A, B, CP);
+          float2 CEN = float2(center_pos.x, center_pos.y);

@@ -308,0 +446,4 @@
+          float flip = angle_cp >= 0.0f ? 1.0f : -1.0f;
+          const float dist_cp_mp = math::distance(CP, MP);
+          const float dist_a_mp = math::distance(A, MP);
+          flip *= dist_a_mp > dist_cp_mp ? -1.0f : 1.0f;

@@ -308,0 +458,4 @@
+          for (const int i : IndexRange(subdivision + 1)) {
+            const float t = theta_step * i + angle_offset;
+            const float x = radius * cos(t);
+            const float y = radius * sin(t);

@@ -1070,0 +1402,4 @@
+    std::reverse(ptd.control_points.begin(), ptd.control_points.end());
+    std::reverse(ptd.temp_control_points.begin(), ptd.temp_control_points.end());
+    ptd.reverse = false;
+    ptd.flip = ptd.flip ? false : true;