Blender Git Loki

Cloth: Fix the shrink feature to be controlled by the sewing spring flag.

Previously it was all over the place: without a vertex group it was
always enabled, and with it it depended on the Self Collision option
due to a likely editing mistake. Now it should be in sync with what
disables the fields in the UI.

The verts->shrink_factor field is changed to just cache the vertex
weight, with the shrink_min/shrink_max interpolation done later.
This is because cloth_apply_vgroup only runs if there are vertex
groups, and thus the factor may not update after property changes.

In addition, bending springs are now also affected by the shrink
factor to avoid visible distortion in object shape at high shrink.