hacks: add NetVips hacks to minimize string allocation

src/Oh.My.Stitcher/NetVips/GObjectHacks.cs

@@ -0,0 +1,38 @@
+using System.Runtime.InteropServices;
+using System.Security;
+using NetVips.Internal;
+using GObjectManaged = NetVips.GObject;
+
+namespace Oh.My.Stitcher.NetVips;
+
+
+internal static unsafe class GObjectHacks
+{
+  internal static void SetProperty(GObjectManaged gObject, string name, ReadOnlySpan<byte> value)
+  {
+    var gvStruct = new GValue.Struct();
+
+    try
+    {
+      GValue.Init(ref gvStruct, 16 << 2);
+      fixed( byte* pValue = value )
+      {
+        GValueHacks.SetString(ref gvStruct, pValue);
+      }
+
+      GObject.SetProperty(gObject, name, in gvStruct);
+    }
+    finally
+    {
+      GValue.Unset(ref gvStruct);
+    }
+  }
+}
+
+internal static unsafe class GValueHacks
+{
+  [SuppressUnmanagedCodeSecurity]
+  [DllImport("libgobject-2.0.so.0", CallingConvention = CallingConvention.Cdecl,
+    EntryPoint = "g_value_set_string")]
+  internal static extern void SetString(ref GValue.Struct value, byte* vString);
+}

src/Oh.My.Stitcher/NetVips/OperationHacks.cs

@@ -0,0 +1,34 @@
+using NetVips;
+using NetVips.Internal;
+using VipsObject = NetVips.Internal.VipsObject;
+
+namespace Oh.My.Stitcher.NetVips;
+
+public static class OperationHacks
+{
+  public static object Call(string operationName, ReadOnlySpan<byte> arg)
+  {
+    var intro = Introspect.Get(operationName);
+    if (intro.RequiredInput.Count != 1)
+      throw new ArgumentException($"unable to call {operationName}");
+
+    nint vop;
+    using( var op = Operation.NewFromName(operationName) )
+    {
+      Introspect.Argument requiredArg = intro.RequiredInput[0];
+      GObjectHacks.SetProperty(op, requiredArg.Name, arg);
+      vop = VipsOperation.Build(op);
+      if (vop == IntPtr.Zero)
+      {
+        VipsObject.UnrefOutputs(op);
+        throw new VipsException($"unable to call {operationName}");
+      }
+    }
+    using (var op = new Operation(vop))
+    {
+      object? result = op.Get(intro.RequiredOutput[0].Name);
+      VipsObject.UnrefOutputs(op);
+      return result;
+    }
+  }
+}