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;

@@ -136,3 +171,4 @@
   ViewOpsData *vod;
 };
 
+static float2 calc_circumcenter_2d(float2 a, float2 b, float2 c)

@@ -143,0 +181,4 @@
+  const float bc1 = ba[1] * dca - ca[1] * dba;
+  const float bc2 = -ba[0] * dca + ca[0] * dba;
+  const float cba = ba[1] * ca[0] - ca[1] * ba[0];
+  const float2 center = float2(a[0] + 0.5f * bc1 / cba, a[1] + 0.5f * bc2 / cba);

@@ -387,0 +578,4 @@
+          const float t = i / float(new_points_num);
+          const float a = t * math::numbers::pi * 2.0f;
+          const float2 uv = float2(math::sin(a), math::cos(a)) * 0.5f + 0.5f;
+          float2 pos = corners[0] * (1 - uv[0]) * (1 - uv[1]);

@@ -924,0 +1366,4 @@
+    if (ptd.type == PrimitiveType::Semicircle) {
+      const float2 start = point_2d_from_temp_index(ptd, ptd.active_control_point_index + 1);
+      const float2 end = point_2d_from_temp_index(ptd, ptd.active_control_point_index - 1);
+      float2 mp = math::midpoint(start, end);