Cleanup: General comment style clean up of graph_edit.c and fcurve.c

No functional changes.

Reviewed By: Sybren A. Stüvel

Differential Revision: http://developer.blender.org/D7850

Fix libmv eigen alignment issues when compiling with AVX support

There would be eigen alignment issues with the custom libmv vector
class when compiling with AVX optimizations. This would lead to
segfaults.

Simply use the std::vector base class as suggested by the old TODO in
the class header.

Reviewed By: Sergey

Differential Revision: http://developer.blender.org/D8968

Fix running tests when building without portable install.

Fix blender constraints automated tests when using march=native

More agressive optimization made the results differ a bit more than the
current error margin would allow. Bump the error margin to be 1e-6
instead of the previous 0.5e-7.

Cleanup: Break early in normal validation

Before we would continue checking normal array values even if we knew
that the normal array would be conidered valid.

Break early to avoid excess iterations and make the code more clear what
it is doing.

No functional changes.

Fix normal computation in opensubdiv when surface derivates are the same

In very rare occations, the returned derivates would be the same. This
would lead to the normal calculation breaking (zero normals).

Solution: Add this edge case to the other corner case checks.

Reviewed By: Sergey

Fix T80899: Crash on editing multiple UVs of multiple different objects at the same time

The issue was two fold.

First something sets the loop element tag and doesn't clear it before
the UV code in question tries to use the tags. Added a sanity clear to
make sure that it operates on a clean tag state.

The next one was that the UV maps in question had quite a few points
that had zero length UV loop edges. This would lead to division by
zero.

Reviewed By: Jeroen Bakker, Brecht

Differential Revision: http://developer.blender.org/D8967

Fix T80899: Crash on editing multiple UVs of multiple different objects at the same time

The issue was two fold.

First something sets the loop element tag and doesn't clear it before
the UV code in question tries to use the tags. Added a sanity clear to
make sure that it operates on a clean tag state.

The next one was that the UV maps in question had quite a few points
that had zero length UV loop edges. This would lead to division by
zero.

Reviewed By: Jeroen Bakker, Brecht

Differential Revision: http://developer.blender.org/D8967

Fix T80651: Material tab disappears

Always return true if the object can have materials. Even if there are
not currently any materials assigned to the object.

Fix material null pointer reference in buttons_context.c

ASAN reported null pointer access when converting a mesh to curves.
Check if the material is a null pointer before trying to use it.