IC/TS: add optional force argument to invalidate() method

[lgritz]

This works the same way as the force argument to invalidate_all() --
if true, invalidates unconditionally; if false, invalidates only if the
file's modifiation date has changes since the cache first opened it.

The default is true for back compatibility, but probably you will often
want false, only invalidating if the file has changed.

[ThiagoIze]

This is causing one of our tests to fail where we load a texture, flip the texture_automip setting, invalidate_all(false), and load the texture again. OIIO has code to check for this in invalidate_all:

            // Invalidate if any unmipped subimage didn't automip but
            // automip is now on, or did automip but automip is now off.
            if (sub.unmipped
                && ((m_automip && f->miplevels(s) <= 1)
                    || (!m_automip && f->miplevels(s) > 1))) {
                all_files.push_back(name);
                break;

but we then a few lines later do a invalidate(f, force); which in this case has force==false and because the file itself didn't change we don't invalidate. I think we want to instead do a invalidate(f, true) since we've already done the checks in this function and know we want an invalidation.

[lgritz]

Oh yes, you're quire right. Fix coming up.