Fixed rewrite_soft_clips() to correctly trim when handling overlapping
STRs.  Previously it could reset s->left/right multiple times,
overwriting the result each time, instead of taking the inner-most
union of the values.

Also tidied up the code to not needlessly compute consensus and STRs
on the first pass.


git-svn-id: svn://svn.code.sf.net/p/staden/code/staden@3724 ba6f2710-ca9f-427c-89d3-5bf716dbc8e1