Fix Uncaught RangeError in canvas tile renderer

[elemoine]

This PR aims to fix #2542.

The bug triggers when a tile layer has an extent and when the layer extent and the view extent do not intersect. In that case the canvas tile layer renderer's prepareFrame function calculates tile ranges and canvas width and height values that cause js errors further down in the function.

This PR fixes the problems by making the canvas tile renderer's prepareFrame clear its canvas and immediately return when the layer extent and view extent don't intersect. We could probably do better and avoid clearing the canvas when that condition happens. I added a FIXME for this.

Please review.

Closes #2542.

[tschaub]

Looks good to me.

I liked the idea of returning false from prepareFrame when there was nothing to render for a layer. I imagine that would save more than avoiding the extra clearRect on the already clear context.

[elemoine]

Ok, you've motivated me to give the "prepareFrame returns false" approach another try. So let's not merge this for now. Thanks!

[elemoine]

New patch attached.

[tschaub]

I didn't run the examples, but the code looks good to me. Assuming all works as expected, please merge. Thanks for the extra work on this one. Not sure how many people will work with layers with limited extents, but I like the idea that we don't compose extra blank pixels in that case.

[elemoine]

Not sure how many people will work with layers with limited extents

It visually looks a bit strange when you zoom in and see that the new tiles covers less ground. I wonder if clipping would be more appropriate for the "limited layer extent" use-case.