VSE: Reduce transform code complexity

Reduce complexity of sequencer transform code by removing recursivity.
This is possible by treating meta strips (mostly) as any other strip and
containing all transform code within SEQ_ functions.

Unfortunately internally meta strips still require special treatment,
but all complexity from code all over transform code seems to be
possible to contain within one function.

Functional change:
Previously adjusting handle of single image strip moved animation.
Now animation is not moved, which is behavior for all other strips.

Reviewed By: sergey, mano-wii

Differential Revision: https://developer.blender.org/D11493

source/blender/makesdna/DNA_sequence_types.h

@@ -167,8 +167,9 @@ typedef struct Sequence {
    * frames that use the last frame after data ends.
    */
   int startstill, endstill;
-  /** Machine: the strip channel, depth the depth in the sequence when dealing with metastrips. */
-  int machine, depth;
+  /** Machine: the strip channel */
+  int machine;
+  int _pad3;
   /** Starting and ending points of the strip in the sequence. */
   int startdisp, enddisp;
   float sat;