Memoryleak in unblockEvents function

[FabioWanner]

In the unblockEvents function, theres the statement: delete this._oldTargets. This does not have the desired effect, causing this._oldTargets to grow indefinitely.

This issue will lead to performance problems and eventually crashes (out of memory). It becomes a problem when working with tousands of layers.

this._oldTargets = {}; solves the problem.