GMUDefaultClusterRenderer uses UIView API off main thread

Calling GMUClusterManager.cluster() from a background queue causes UIKit violations due to GMUDefaultClusterRenderer internally accessing UIView and CALayer properties off the main thread.

Steps to Reproduce:
1. Use GMUDefaultClusterRenderer in a Swift app
2. Call .cluster() from a background queue
3. Observe runtime warnings:
• -[UIView bounds] must be used from main thread only
• -[UIView layer] must be used from main thread only

Suggested Fix:
Ensure all UIKit access in GMUDefaultClusterRenderer is performed on the main thread, or document that .cluster() must be called on the main thread when using GMUDefaultClusterRenderer.