[Bf-codereview] Add BMesh and WM symmetrize operators (issue 6618059)

[NicholasBishop at gmail.com]

> Guess this todo and the one a few lines lower is not difficult to fix?
Oops, forgot about this. Fixed.

> I think these values are quite high, mirror for transform and
particles uses
> 0.00002f (meshtools.c MOC_THRESH). For Sintel's head model I remember
Campbell
> set it to this low value because it didn't work with small detail. Not
sure if
> it should be user adjustable but probably it should be smaller?
Changed to 0.00002f.

> I have a feeling there is a corner case where r = 0 and lambda is used
> uninitialized. I didn't try to recreate it, but it probably exists.
Added initialization to zero for lambda.

> This could call ghash_lookup without ghash_haskey first, will just
return NULL.
Removed the unnecessary BLI_ghash_haskey() calls.

> It would be slightly more clear if 0 is replaced with
BMO_SYMMETRIZE_NEGATIVE_X
> here.
Fixed.

I made one additional change: symmetrical edges with no faces attached
were getting deleted. Pulled out some code from
symm_split_asymmetric_edges() into a new symm_edge_crosses_axis()
function, and in symm_kill_unused() check that the edge is not
symmetric.

Thanks for reviewing!

https://codereview.appspot.com/6618059/