Animation: action mirror RNA API using pose contents

This adds a new RNA method `Action.flip_with_pose(ob)` to flip the
action channels that control a pose.
The rest-pose is used to properly flip the bones transformation.

This is useful as a way to flip actions used in pose libraries,
so the same action need not be included for each side.

Reviewed By: sybren

Ref D10781

source/blender/makesrna/intern/rna_action_api.c

@@ -41,10 +41,32 @@
 #  include "DNA_anim_types.h"
 #  include "DNA_curve_types.h"
 
+static void rna_Action_flip_with_pose(bAction *act, ReportList *reports, Object *ob)
+{
+  if (ob->type != OB_ARMATURE) {
+    BKE_report(reports, RPT_ERROR, "Only armature objects are supported");
+    return;
+  }
+  BKE_action_flip_with_pose(act, ob);
+
+  /* Only for redraw. */
+  WM_main_add_notifier(NC_ANIMATION | ND_KEYFRAME | NA_EDITED, NULL);
+}
+
 #else
 
-void RNA_api_action(StructRNA *UNUSED(srna))
+void RNA_api_action(StructRNA *srna)
 {
+  FunctionRNA *func;
+  PropertyRNA *parm;
+
+  func = RNA_def_function(srna, "flip_with_pose", "rna_Action_flip_with_pose");
+  RNA_def_function_ui_description(func, "Flip the action around the X axis using a pose");
+  RNA_def_function_flag(func, FUNC_USE_REPORTS);
+
+  parm = RNA_def_pointer(
+      func, "object", "Object", "", "The reference armature object to use when flipping");
+  RNA_def_parameter_flags(parm, PROP_NEVER_NULL, PARM_REQUIRED);
 }
 
 #endif