Constrain center in zoomify example
Merge pull request #2847 from elemoine/zoomify-example

Constrain center in zoomify example
Assert images have a size before canvas rendering

IE does not know the size of a SVG (yet) when adding icons to the canvas.
This commit asserts that images have a valid width & height, before adding
them to the canvas.
To enable SVG icons on IE, one must provide a size in `ol.style.Icon`.
Fix SVG hit detection on IE

IE does not know the size of SVG icons. When creating a 'hit detection icon',
OpenLayers will fail, as the canvas won't accept a 0x0 sized image.
This commit ensures the 'hit detection icon' is creating using the same size
as the original image. Note that one must provide a size in `ol.style.Icon`
for this to have any effect on IE.