_project called on removed layer in canvas renderer

[perliedman]

While looking at #5093, I discovered that current master has a problem when layers are removed from the canvas renderer.

When removing a layer, the removed layer's _project method is sometimes called even though it's removed, causing an exception since the layer's this._map is not set.

Example is this modified playground from #5093: http://playground-leaflet.rhcloud.com/firu/edit?html,output

Click the map twice, the second time you will get an exception and the map will not update properly.

I believe the problem lies in the _requestRedraw's asynchronous behaviour: in this particular case, the redraw is postponed until after the fitBounds call, so the layer has not yet been removed from the renderer's _layers (this is done in draw when the clearflag is set).

I believe this is caused by the refactor in #5054.

[perliedman]

I'm in the process of addressing this issue over here: https://github.com/perliedman/leaflet/tree/wip-canvas-clear

This involves a bit more refactoring of the canvas rendering, so we'll have to test this somewhat thoroughly before merging.