* Enable nullable annotations
* Remove unused Jetbrain annotations
* Ensure system using statements are first
* Improve nullability annotations
* Annotate encryptionDictionary is non-null when IsEncrypted is true
* Disable nullable for PdfTokenScanner.Get
* Improve nullability annotations for ObjectLocationProvider.TryGetCached
* Revert changes to RGBWorkingSpace
* Update UglyToad.PdfPig.Package with new framework targets (fixes nightly builds)
most of the cross-reference code is the earliest code in the project and hasn't been revisited since then. the issue #88 has been reopened due to a bug with brute-force searching so this tidies up the code in this area ahead of trying to fix the bug.
when brute force searching for the start of the cross-reference table (xref) we read 5 byte buffers, previously if the 'x' of 'xref' was the first character of the buffer we skipped it. this checks when 'x' is the first character of the buffer.
if we are reading a cross reference offset which contains a number we assumed it was a stream object. if it's not we now brute-force the entire file looking for an 'xref' token. this should be combined with a search for cross-reference streams and should run when we read neither the numeric token or an 'xref' token but for now this fixes the observed issue.
also adds number of images to the page api to prevent consumers needing to enumerate.
if the %pdf version header comment is offset from the start of the file the cross reference offsets will also be wrong by that amount. this change updates the cross reference location logic to use the offset from the located version header.
if the previous cross-reference location points to an offset outside the file size we skip it.
also makes cid font factory more resilient by skipping missing descriptors.
to make the project more useful and expose more usable classes we're rearchitecting in the following way. code used to read fonts from external file formats like truetype, adobe font metrics (afm) and adobe type 1 fonts are moving to a new project which doesn't reference most of the pdf logic. the shared logic is moving to a new flat-structured project called core. this is a sort-of onion type architecture, with core being the... core, fonts being the next layer of the onion, pdfpig itself the next. this will then support additional libraries/projects as outer layers of the onion as well as releasing standalone version of the font library as pdfbox does with fontbox.
previously if the cross reference did not exist at exactly the provided offset we'd immediately throw, now we assume we can read a few more tokens to find the xref table or stream start. this won't work in the case where the provided offset is past the start of the table or nowhere near the table but in those cases there's not much we can do. there's some more work to do to provide a fallback xref parser which finds the xref tables and streams using a brute-force scan of the whole document.
