GPv3: Draw on back.

@@ -177,0 +177,4 @@
+  static void create_blank_curve(bke::CurvesGeometry &curves, const bool on_back)
+  {
+    if (on_back) {
+      const bke::CurvesGeometry TempCurve = bke::CurvesGeometry(curves);

@@ -177,0 +192,4 @@
+                                     bke::AttrDomain::Point,
+                                     {},
+                                     {},
+                                     OffsetIndices<int>(Array<int>({0, curves.points_num() - 1})),

@@ -177,0 +205,4 @@
+                                     IndexMask(IndexRange(1)),
+                                     attributes);
+    }
+    else {

@@ -193,0 +199,4 @@
+      return;
+    }
+
+    const bke::CurvesGeometry temporal_copy = bke::CurvesGeometry(curves);

@@ -193,0 +241,4 @@
+    const int active_curve = curves.curves_range().first();
+    const int last_active_point = curves.points_by_curve()[active_curve].last();
+
+    const bke::CurvesGeometry temporal_copy = bke::CurvesGeometry(curves);

@@ -238,2 +313,4 @@
     const float start_opacity = ed::greasepencil::opacity_from_input_sample(
         start_sample.pressure, brush_, scene_, settings_);
+    Scene *scene = CTX_data_scene(&C);
+    const bool on_back = (scene->toolsettings->gpencil_flags & GP_TOOL_FLAG_PAINT_ONBACK);

@@ -405,2 +486,4 @@
     const float opacity = ed::greasepencil::opacity_from_input_sample(
         extension_sample.pressure, brush_, scene_, settings_);
+    Scene *scene = CTX_data_scene(&C);
+    const bool on_back = (scene->toolsettings->gpencil_flags & GP_TOOL_FLAG_PAINT_ONBACK);

@@ -408,3 +491,4 @@
     bke::CurvesGeometry &curves = drawing_->strokes_for_write();
     bke::MutableAttributeAccessor attributes = curves.attributes_for_write();
 
+    int active_curve = on_back ? curves.curves_range().first() : curves.curves_range().last();

@@ -439,2 +525,2 @@
-    curves.offsets_for_write().last() = curves.points_num();
-    const IndexRange curve_points = curves.points_by_curve()[curves.curves_range().last()];
+    extend_curve(curves, on_back, new_points_num);
+    curve_points = curves.points_by_curve()[active_curve];

@@ -612,3 +700,2 @@
 {
-  const int stroke_index = drawing.strokes().curves_range().last();
-  const IndexRange points = drawing.strokes().points_by_curve()[stroke_index];
+  Scene *scene = CTX_data_scene(&C);

@@ -613,2 +701,2 @@
-  const int stroke_index = drawing.strokes().curves_range().last();
-  const IndexRange points = drawing.strokes().points_by_curve()[stroke_index];
+  Scene *scene = CTX_data_scene(&C);
+  const bool on_back = (scene->toolsettings->gpencil_flags & GP_TOOL_FLAG_PAINT_ONBACK);

@@ -645,0 +746,4 @@
+  const int active_curve = curves.curves_range().first();
+  const int last_active_point = curves.points_by_curve()[active_curve].last();
+
+  const bke::CurvesGeometry temporal_copy = bke::CurvesGeometry(curves);

@@ -193,0 +210,4 @@
+
+    bke::MutableAttributeAccessor attributes = curves.attributes_for_write();
+
+    copy_attributes_group_to_group(src_attributes,

@@ -193,0 +208,4 @@
+      offsets[i + 1] = offsets[i] + 1;
+    }
+
+    bke::MutableAttributeAccessor attributes = curves.attributes_for_write();

@@ -193,0 +195,4 @@
+    if (!on_back) {
+      const int num_old_points = curves.points_num();
+      curves.resize(curves.points_num() + 1, curves.curves_num() + 1);
+      curves.offsets_for_write().last(1) = num_old_points;

@@ -646,0 +782,4 @@
+           i < curves.points_num() - rem_points_num;
+           i++)
+      {
+        span_data[i] = span_data[i + rem_points_num];