Improve pytmx Pygame Loader, Unit Test and Cleaner Typing

[JaskRendix]

This pull request introduces a range of improvements:

• improved smart_convert logic: adds a new preserve_alpha_flag parameter to optionally retain per-pixel alpha formatting, even when no transparency is detected, replaces manual pixel counting with the shared helper function has_transparency() for cleaner and more consistent transparency detection, introduces count_colorkey_pixels() to estimate colorkey coverage, deciding more intelligently when to apply RLEACCEL, and strengthens fallback behavior when mask operations fail
• adds preserve_alpha_flag flag allows external tools or workflows to retain original surface format when alpha is present but not visibly used
• adds complete type hints throughout the module, including accurate annotations for callables
• switches all string formatting to f-strings for cleaner syntax and better readability
• adds new unit tests for nuanced transparency cases, colorkey matching behavior, performance benchmarks, and zero-sized surfaces, all tests pass before and after the changes, ensuring consistency and reliability
• refactors tileset and layer name lookups to use safer and more Pythonic patterns, with better error messages for unexpected types
• optimizes the simplify() function for more efficient geometry handling
• adds a protective check to ensure tiles are square before attempting diagonal flips, preventing misaligned transformations
• standardizes error messages across utility functions for improved clarity

[pnearing]

I've forked the project and uploaded to pypi as pytmx-ng, I'd love to have your code in there, can you open a pull request here: https://github.com/pnearing/pytmx-ng ? Thanks, and I hope this helps you. (this goes for your other pull requests too)