Fix Interop.Gdi32.StartDoc p/invoke (#76569)

StartDoc takes a pointer to a DOCINFO struct. We had DOCINFO defined as a class on the managed side such that built-in marshalling would marshal it as a pointer. With the switch to custom marshalling via NativeMarshalling, we ended up marshalling the struct that is the native representation rather than a pointer to it.

This switches the DOCINFO to a struct and updates the parameter to StartDoc to be in, such that it should be marshalled as a pointer to the struct by both generated and built-in marshalling.

src/libraries/System.Drawing.Common/src/Interop/Windows/Interop.Gdi32.cs

@@ -52,7 +52,7 @@ internal static partial int StartDoc(
 #if NET7_0_OR_GREATER
             [MarshalUsing(typeof(HandleRefMarshaller))]
 #endif
-            HandleRef hDC, DOCINFO lpDocInfo);
+            HandleRef hDC, in DOCINFO lpDocInfo);
 
         [LibraryImport(Libraries.Gdi32, SetLastError = true)]
         internal static partial int StartPage(
@@ -179,14 +179,16 @@ internal unsafe struct BITMAPINFO_FLAT
         [NativeMarshalling(typeof(Marshaller))]
 #endif
         [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Auto)]
-        internal sealed class DOCINFO
+        internal struct DOCINFO
         {
             internal int cbSize = 20;
             internal string? lpszDocName;
             internal string? lpszOutput;
             internal string? lpszDatatype;
             internal int fwType;
 
+            public DOCINFO() { }
+
 #if NET7_0_OR_GREATER
             [CustomMarshaller(typeof(DOCINFO), MarshalMode.ManagedToUnmanagedIn, typeof(Marshaller))]
             public static class Marshaller