Fix T89564: Spline IK breaks when it is far away from the world origin

The isect_line_sphere algorithm became very imprecise when the line and
the sphere were reasonably far away from the world origin.

This would lead to no intersections being reported even if there was a
guaranteed intersection (line crossing from inside the sphere to the
outside).

To fix this we now use the secant root finding method to get an
intersection point. This is much more stable and robust it seems.

Merge branch 'blender-v3.0-release'

Fix: The bounding box gizmo breaks if transform pivot is set to cursor

The bounding box transform code assumed that the pivot would always be
the sequence object transform center.

Rework the code so that this assumption is true even if the general
transform pivot is set to be the 2D cursor.

Fix T92355: Quadriflow crashes with zero length edges

Add a check for zero length edges to the manifold check as quadriflow
doesn't handle meshes with these.

Merge branch 'blender-v3.0-release'

Fix install paths for blender thumbnailer when not building a portable install

When doing a non portable build of blender, the executable
blender-thumbnailer would be installed in two locations:
/usr/bin/
/usr/

While cleaning up, also make the blender thumbnailer dll optional on
windows to bring the logic in line with what it is on linux and mac.

Reviewed By: Campbell Barton, Ray molenkamp

Differential Revision: http://developer.blender.org/D13014

Fix T92355: Quadriflow crashes with zero length edges

Add a check for zero length edges to the manifold check as quadriflow
doesn't handle meshes with these.

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.