Bug 1073401 - Fix hit testing regression for invisible and detached objects

Includes a test case preventing future regressions.
Bug 1073877 - Fix hit testing regression for objects on depths below the first clipping mask

The loop that eliminates clipping-hidden children lazily creates a copy of the children array only if a clipping mask is encountered. It didn't copy over all children below said clipping mask, so those were never hit-tested.
Merge pull request #1783 from tschneidereit/master

Fix hit testing regressions