layer container and viewport elements can have same ids for multiple maps

[tschaub]

If a map is created with an element that doesn't have an id (or if the map is created with no element at all), the layer container and viewport elements will be given ids like _OpenLayers_Container and _OpenLayers_ViewPort. Two maps created in this way will have the same ids for these elements.

[tschaub]

try to create unique ids

[tschaub]

Tests pass. Thanks for any review.

[crschmidt]

Tests pass, fixes a buggy behavior. Would be curious to know what turned this up; I don't think anything in OpenLayers uses these? Assume it was a problem for some other tool.

I doubt that this is the only case where duplicate ids can show up; we've not been super-careful about this in the past, in my experience.

[tschaub]

(In [9704]) Use a unique id for the map viewport. r=crshmidt (closes #2259)