Don't check hover if hovering features are empty

[utilmind]

Otherwise, an error may occur in the inBounds() function when it checks if some point is within the bounds but the bounds are empty.

This fixes possible issue with glify.lines() layers.

[utilmind]

New patch contains a fix for issue #182. So hover handlers now correctly implemented for glify.lines(). Plus added some performance optimizations, like cached bounding rectangle with all features, etc.

[utilmind]

Actually that check, if (inBounds(e.latlng, instance.incBounds(sensitivityHover))) can be removed completely.

In my particular implementation users working only on the part of map filled with features, so we don't need to check whether mouse hovers the rectangle with features. The features are certainly always in view in my case.

So maybe it worth to add another additional option, something like "checkHoverBounds" and do that inBounds() check only when option is turned on (true).

So for my particular implementation this inBounds() is not necessary. Personally I would remove it completely. However I fixed it only because it was already part of the code. Now it performs properly.