Adds PdfPageGraphics.Clear method.
Features
- Added PdfPageGraphics.Clear method.
Fixes
- 4bit PNG images with alpha channel are not drawn correctly on the PDF page.
- Checkboxes with /Opt array return incorrect export value and cannot be checked by value.
- The appearance of stamp annotations changes when annotation position/size changes.
- Some PDF files are not merged correctly.
- Some damaged PDF files cannot be loaded.