Need to add helpers for validating Graphics rendering

There are two components to this. The first is relatively easy- we need helpers and some examples to view GDI calls generated when rendering to a Graphics object. The second is more complicated- we need to be able to look at GDI+ records.

Recording and playing back GDI records from a Graphics object can be done in a few ways:

1. We can create a Graphics object around a Metafile HDC we create and just look at that HDC the way we’re already doing it.
2. We can create a System.Drawing MetaFile around a stream and create a Graphics object from that to record to. You can then grab the HENHMETAFILE from the MetaFile and enumerate the way we already are.

Enumerating the GDI+ records is more complicated as the record headers and parsing need to be created from scratch by following the EMF+ specification.

I’ve started fiddling with the lower level APIs with this here: https://github.com/JeremyKuhne/WInterop/blob/main/src/Tests/WInterop.Tests/GdiPlus/Metafiles.cs#L65 This code can be used to find the equivalent System.Drawing entry points.

The EMF+ specification: https://docs.microsoft.com/openspecs/windows_protocols/ms-emfplus/5f92c789-64f2-46b5-9ed4-15a9bb0946c6

[willibrandon]

I can now inspect the PointData in the EmfPlusDrawLines record. I understand what you mean about the process being slow.

@drawLines->Count.Should().Be(2); // Number of points

MetafilePlusPoint from = @drawLines->GetPoint(0);
MetafilePlusPoint to = @drawLines->GetPoint(1);
from.X.Should().Be(1);
from.Y.Should().Be(1);
to.X.Should().Be(3);
to.Y.Should().Be(5);

[JeremyKuhne]

@willibrandon Enumerating the top-level records in EMF+ is relatively easy as GDI+ gives you a record enumerator (exposed in System.Drawing.Imaging.Metafile). Getting the details out of each record takes a little bit of work to get started as you have to sequentially scan as there aren’t offsets to let you skip to the data you care about. I started playing with this in the WInterop project I linked above (the tests in the link above show how the data presents itself when viewed as a standard EMF or through the EMF+ enumerator). It can be used freely as a base/reference to doing a more complete parser. I intend to keep expanding the implementation there, but I don’t have an expected timeframe.