Fix extra tile usage

[IvanSanchez]

Should fix #4180

Added a new viewprereset map event, run before the viewreset event.

So the sequence for when fadeAnimation is false is: unload tiles on viewprereset, then move the map to the proper viewport, then load tiles on viewreset. This makes PhantomJS a viable option for unit-testing tile loading counts.

Unit tests in graphical browers do fail as of now, though.

[yohanboniface]

Sorry to insist a bit, but as per my understanding we don't need scale: if the map size is 600px at zoom 10 it will still be 600px at zoom 11 or 17 or 17.358, isn't it? Or am I missing something?

[IvanSanchez]

@yohanboniface You're missing fractional zoom. In those cases, 0.5 < scale < 2. See the debug/tests/tile-bounds.html file.

e.g. if the zoom level is 17.348, and the map is 600px, then there are less than 600px worth of tiles (and less tiles are loaded) because tile pixels are bigger.

[mourner]

Added a new viewprereset map event, run before the viewreset event.

That's just asking for double brainmelt in future. :)

[IvanSanchez]

That's just asking for double brainmelt in future. :)

@mourner Actually, I don't think so. Layers should clear everything up on viewprereset, then add everything back again in viewreset. Makes sense.

God, I want leafdoc merged in so I can document these kind of things inline right away.

[yohanboniface]

I want leafdoc merged in so I can document these kind of things inline right away.

Why are we not there yet?

[IvanSanchez]

I think this PR introduces some kind of race condition on flyTo animations that makes tiles to not be pruned when switching levels, likely due to https://github.com/Leaflet/Leaflet/pull/4193/files#diff-f1e6be67599c594731fff6191c710420L655 - must find a way to trigger a pruning once in a while.

[hyperknot]

Some debugging on tile-events.html:

1. The load count for TileLayers is not reflecting the actual img requests from the browser tools. In my case: grid = 20, postiron = 40, img requests = 20.
2. It is happening when grid is added to map. If map.addLayer(grid); is commented out, the tileLayer values are correct (except tileloadstart).
3. tileloadstart does not fire for initial loading of tileLayer. Adding setTimeout to gridLayer fixes tileLayer.

   setTimeout(function(){
            this.fire('tileloadstart', {
                tile: tile,
                coords: coords
            });
        }.bind(this));

[hyperknot]

#4145 also OK with this one:
http://jsbin.com/ratoli/9/edit?html,js,output