VSE: Implement the bounding box (xform) tool in the seq preview window

Make the "xform" tool/gizmo available for strip transformations in the
sequencer preview window.

Because of the amount of hacks needed to make the gizmo work nicely with
multiple strips at the same time, it was decided to only show the
translate gizmo when multiple strips are selected.

This is because the transforms with multiple strips would appear buggy
because of our lack of shearing support in the transform system.
There is also currently no way to properly sync the gizmo drawing with
the transform when using multiple strips.

Reviewed By: Richard Antalik, Campbell Barton

Differential Revision: http://developer.blender.org/D12729

VSE: Free animation strip data if they are not visible

Previously we would only free animation strip data when doing final
renders. If not doing a final render or simply just playing back videos
in the VSE, we would not free decoders or non VSE cache data from the
strips.

This would lead to memory usage exploding in complex VSE scenes.

Now we instead use the dumb apporach of freeing everything that is not
currently visible.

Cleanup: Make anim_getnew in the VSE less confusing

It was using dummy image buffers to indicate if an animation container
could be initialized or not.

Use booleans instead.

VSE: Free animation strip data if they are not visible

Previously we would only free animation strip data when doing final
renders. If not doing a final render or simply just playing back videos
in the VSE, we would not free decoders or non VSE cache data from the
strips.

This would lead to memory usage exploding in complex VSE scenes.

Now we instead use the dumb apporach of freeing everything that is not
currently visible.

Fix session uuid ghash comparison return value

Because of legacy reasons (C string compare function returning 0 when
strings are equal), the ghash compare function is expected to return
false when hashes are equal.

VSE: Draw active strips with a different color in the preview window

Cleanup and remove SEQ_ALL_BEGIN macro

We now use a for_each function with callback to iterate through all sequences in the scene.

This has the benefit that we now only loop over the sequences in the scene once.
Before we would loop over them twice and allocate memory to store temporary data.

The allocation of temporary data lead to unintentional memory leaks if the code used returns to exit out of the iteration loop.
The new for_each callback method doesn't allocate any temporary data and only iterates though all sequences once.

Reviewed By: Richard Antalik, Bastien Montagne

Differential Revision: http://developer.blender.org/D12278

FFMPEG: Fix building with older versions that need FFMPEG_USE_DURATION_WORKAROUND

VSE: Flush audio encode after finishing video export

We didn't flush audio after encoding finished which lead to audio
packets being lost.

In addition to this the audio timestamps were wrong because we
incremented the current audio time before using it.

Reviewed By: Richard Antalik

Differential Revision: http://developer.blender.org/D11916

FFMPEG: Fix building with older versions that need FFMPEG_USE_DURATION_WORKAROUND