Fixed 'bounceAtZoomLimits' when you don't start pinching from max\min level

When you enable `bounceAtZoomLimits`, it works correctly only if you start pinching from the maximum or minimum level (affected all Leaflet version that supports such feature)

To reproduce the bug:
1- set `bounceAtZoomLimits`, set minLevel of the map to 10, and maxLevel to 15
2- pinch to level 11, and now pinch out how far you can.
3- observe how you are able to pinch out far beyond the minimum level (however when you release the touch the map zoom will be correctly set to the right minimum level)

The fix calculate the proper current zoom level, and don't use `map.getZoom()` that is updated only when user actually finished the pinch releasing touch.