Cleanup: split sequencer.c file

Move most functions into separate files.

File sequencer.c is reserved for alloc, free and duplicating functions.

There should be no functional changes.

source/blender/sequencer/intern/utils.h

@@ -0,0 +1,41 @@
+/*
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ *
+ * The Original Code is Copyright (C) 2004 Blender Foundation.
+ * All rights reserved.
+ */
+
+#pragma once
+
+/** \file
+ * \ingroup sequencer
+ */
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+struct Scene;
+struct anim;
+
+bool sequencer_seq_generates_image(struct Sequence *seq);
+void seq_open_anim_file(struct Scene *scene, struct Sequence *seq, bool openfile);
+struct Sequence *BKE_sequence_metastrip(ListBase *seqbase /* = ed->seqbase */,
+                                        struct Sequence *meta /* = NULL */,
+                                        struct Sequence *seq);
+
+#ifdef __cplusplus
+}
+#endif