Determine image size on load

[tschaub]

This makes it so the image size does have to be provided for static image sources. If not provided, the size is determined when the image loads.

If a ol.source.ImageStatic is constructed without an imageSize option, the source is given the "loading" state and image loading is triggered. When the image loads, the image resolution is calculated, and the source is given the "ready" state.

Fixes #2767.

[elemoine]

If no image size was provided in the options passed to the constructor then undefined will be passed the parent constructor for the resolutions option. Looking at the code I don't think this a problem, because ol.source.ImageStatic doesn't use ol.source.Image#findNearestResolution. For clarity it may make sense to not pass resolutions to the parent constructor.

[elemoine]

It looks good to me. I just added three pretty minor comments.

Also, with this change, the ImageStatic constructor itself loads the image if no image size was provided in the constructor options. Initially the renderer was the only one responsible for loading images, which means images were loaded at only one place in the code. But I'm wondering why we need that the constructor load the image? Why don't we let the renderer do it as previously? This would avoid the handling of source states in ImageStatic.

[tschaub]

But I'm wondering why we need that the constructor load the image?

Indeed we don't. See 4979609.

Thanks for the review.

[elemoine]

Please merge.