PdfiumNet

Modified PdfiumViewer, adapted .Net Standard 2.1/.Net 5.0/.Net 6.0.

Primary modifications:

• REMOVED all WinForm related codes
• Substituted GDI+ with SKIA
• Minor modifications on rendering API, the code should explain itself
• Compatiable with the original version of Pdfium from Google and the modified version which hava an export of AddInitializationCount