Create a .NET 6 code path for GlobalMemory

[DmitryGolubenkov]

Summary of the PR

• Added a net6.0 target to Silk.NET.Core.csproj
• Added a new unsafe struct NativeMemoryPointer inside a preprocessor NET6_0_OR_GREATER
• Modified GlobalMemory.Free and GlobalMemory.Allocate to use the new struct.

Related issues, Discord discussions, or proposals

#597

Further Comments

I don't know if this PR require testing or contain breaking changes as I am not aware of any possible problems.

I had to add unsafe to GlobalMemory.Free to use NativeMemory because that is required. And this change will affect .NET 5 and .NET Standart versions even if this change is only for .NET 6.
Also: maybe it is better to use NET6_0_OR_GREATER directive in GlobalMemory.Free? Because NET6_0 targets only one version, and I think next year someone will have to do this change anyway.

NativeMemoryPtr requires minimum .NET 6 too. Also I had to make some convertions line (nint) and (nuint) to be able to use NativeMemory.

I think that GlobalMemory.Allocate requires a little work, because the preprocesor directives now are.. Confusing.
Now it is:
#if NET6_0
#elif !NET5_0
#else

I think it is better to make
#if NET6_0
#elif NET5_0
#else
and make according changes to the code. Only to make the code more readable.

[HurricanKai]

Thank you for you contribution!
I agree that the preprocessors in Allocate are confusing, if possible do change that.
Also, there are a lot of places in Silk.NET.Core where NET5_0 is used, and those codepaths should be available on NET6_OR_HIGHER as well, if possible please change those as well.

[DmitryGolubenkov]

Should I make another PR to make these changes to whole Silk.NET.Core? Or just commit everything to this?

[HurricanKai]

Committing to this is fine

[Perksey]

Thanks again for the shear volume of contributions as of late! Really appreciate it :)