support mapViewDidFinishRenderingMap:fullyRendered:NO

[jfirebaugh]

Followup to #1431.

What remains unimplemented is support for fullyRendered:NO -- we currently do not surface tile errors in a way that made this easy to implement, so I'm punting for now.

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[1ec5]

Prerequisite for #2775.

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[mkrussel77]

We typically create VectorSources for data that changes and may or may not have anything visible.

Currently if there is a failure to download the tiles, it just looks to the user that there is nothing happening.

Being able to report that there has been an error would let them know that there might be something there but we are unable to display it to them.

Ideally it would be good to know which source had problems so we know if it is one of our sources or a source that was part of the style.

[zugaldia]

/fyi: @tmpsantos

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This issue has been automatically detected as stale because it has not had recent activity and will be archived. Thank you for your contributions.