Refactored PNG loading and SFont handling:

- Improved memory management and error handling in _loadFromPNG, oslLoadSFontFile, oslDeleteSFont, oslGetSFontTextWidth, and oslSFontDrawText.
- Added proper resource cleanup in case of failure.
- Used const correctness where appropriate.
- Simplified code structure and improved readability.
- Added bounds checking and null checks for safer code.