improve grid performance with similarly sized objects

[gavin-ts]

Summary

If the object sizes don't vary much, we don't need to try as many threshold values.
If the fastLayout result is perfect, we don't need to search.

Details

• adds grid_large_checkered test (which would previously timeout)
• fixes performance issues in Feature Request: QR code #1302 leading to timeouts
• refactor with constants
• check standard deviation of sizes when determining how many threshold values to try
• adds a skip limit, for if there are so many options even checking to skip them takes too long
• improves debug logging

new test