Cleanup: use const mesh arg to BM_mesh_bm_from_me

Needed for D3966

source/blender/blenkernel/BKE_mesh.h

@@ -83,7 +83,7 @@ extern "C" {
 /* *** mesh.c *** */
 
 struct BMesh *BKE_mesh_to_bmesh_ex(
-        struct Mesh *me,
+        const struct Mesh *me,
         const struct BMeshCreateParams *create_params,
         const struct BMeshFromMeshParams *convert_params);
 struct BMesh *BKE_mesh_to_bmesh(

source/blender/blenkernel/intern/mesh.c

@@ -718,7 +718,7 @@ Mesh *BKE_mesh_copy(Main *bmain, const Mesh *me)
 }
 
 BMesh *BKE_mesh_to_bmesh_ex(
-        Mesh *me,
+        const Mesh *me,
         const struct BMeshCreateParams *create_params,
         const struct BMeshFromMeshParams *convert_params)
 {

source/blender/bmesh/intern/bmesh_mesh_conv.c

@@ -193,7 +193,7 @@ static BMFace *bm_face_create_from_mpoly(
  * \warning This function doesn't calculate face normals.
  */
 void BM_mesh_bm_from_me(
-        BMesh *bm, Mesh *me,
+        BMesh *bm, const Mesh *me,
         const struct BMeshFromMeshParams *params)
 {
 	const bool is_new =

source/blender/bmesh/intern/bmesh_mesh_conv.h

@@ -52,7 +52,7 @@ struct BMeshFromMeshParams {
 	int64_t cd_mask_extra;
 };
 void BM_mesh_bm_from_me(
-        BMesh *bm, struct Mesh *me,
+        BMesh *bm, const struct Mesh *me,
         const struct BMeshFromMeshParams *params)
 ATTR_NONNULL(1, 3);