feat(core): add GfxObjMesh.Build multi-surface mesh extractor

src/AcDream.Core/Meshing/GfxObjSubMesh.cs

@@ -0,0 +1,12 @@
+using AcDream.Core.Terrain;
+
+namespace AcDream.Core.Meshing;
+
+/// <summary>
+/// One sub-mesh of a GfxObj: a vertex+index buffer that uses a single Surface.
+/// A GfxObj with multiple surfaces produces multiple sub-meshes.
+/// </summary>
+public sealed record GfxObjSubMesh(
+    uint SurfaceId,
+    Vertex[] Vertices,
+    uint[] Indices);