Retarget aghacks with dotnet standard 1.3 as a minimum

[Marv51]

I cleaned up the compatibility classes and attributes in AgHacks.cs, assuming that .net standard 1.3 is currently the lowest supported version. This removes any compatibility hacks from the .net standard 2.0 build.

This fixes #102, #106 and #76 (or at least it should)

It builds and all the tests pass, I'm not familiar with what other kind of testing is required to make sure all supported versions work well with this change.

Maybe @max-brightspark knows something about this? He changed the IConvertible implementation in 2018.

[dejx]

Looks good, another approach could be by using this interfaces only on silverlight platform, but this means targeting netstd 1.3 in all other projects.
E.g. https://github.com/dejx/PdfSharpCore/pull/1/files

[Marv51]

I believe the minimum requirement for this repo is .net standard 1.3 (at least it was broken on anything below that).

So I think #if SILVERLIGHT will be always false. BrowsableAttribute on the other hand is not available in standard 1.3 so the compatibility implementation is still needed.

[dejx]

Cool. All good with me. @ststeiger when can we expect new package? :)

[ststeiger]

@Marv51: Merged.
@dejx: The package should be automatically rebuilt as we speak.

Why did you remove
uint System.IConvertible.ToUInt32(IFormatProvider provider)
etc.

That was a fix for a bug that I ran into, too.
Is this no longer required with a higher netstandard-version, conflicting ?
Or did you just delete it ?

[ststeiger]

Here's the corresponding issue for the change in IConvertible:
#2

and the corresponding pull-request:
#8

[Marv51]

The compatibility hack (I believe) works like this:
If the framework provides System.IConvertible, then the interface declaration references that.
If System.IConvertible is not available (like on silverlight) AgHacks provides a PdfSharp.IConvertible interface.

Because the #if's in the AgHacks where not working correctly for pdfsharpcore, builds with both PDFSharp.IConvertible and System.IConvertible were built. Leading the compiler to choose the PDFSharp version of the interface eventhought the System one is available.

IConvertible is available very broadly today (https://docs.microsoft.com/en-us/dotnet/api/system.iconvertible), so I removed the compatibility version of the interface (Leading the other code to always implement System.IConvertible)

[ststeiger]

OK, we'll see if it works, if it does, all fine with me.