iText Core Table Rendering Optimization: From 5 Minutes to 7 Seconds

Apryse engineer Guust optimized iText Core's table rendering performance. By avoiding repeated border collapse calculations and unnecessary tagging overhead, rendering time for a 50,000-cell table dropped from 5 minutes to 7 seconds. Optimizations focused on the `CollapsedTableBorders#getVerticalBorder` function and tag processing, significantly improving performance through caching, removing redundant function calls, and adding tags in bulk. This optimization is included in iText Core 9.1.0.