onDidBecomeIdle is not called when adding a VectorSource with an invalid URL

[julianrex]

Reproduced on iOS, though I believe this is a lower-level issue.

Steps to reproduce

1. Add an MGLVectorTileSource to the style, with an invalid configuration URL (or valid config but without access)
2. Move the map
3. mapViewDidBecomeIdle is not called.

Note: discovered during testing of #15744

Expected behavior

onDidBecomeIdle should be called if sources with invalid URLs are non-recoverable.

Actual behavior

In Map::Impl::onDidFinishRenderingFrame the renderMode is always Partial, meaning that onDidBecomeIdle is not called.

Configuration

Tested as of: 6965fc3

[stale]

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