Refactor RenderMesh() to now purely render what's specified

* So RenderMesh doesn't pick up anything implicitly from the current
  event, log, pipeline state etc - everything it needs is explicitly
  provided by the config parameters (note this might include a buffer
  generated by postvs data fetching, but the implementation now doesn't
  need to care or treat it as a special case.

renderdocui/Interop/FetchInfo.cs

@@ -362,9 +362,10 @@ namespace renderdoc
     public struct MeshFormat
     {
         public ResourceId idxbuf;
+        public UInt32 idxoffs;
         public UInt32 idxByteWidth;
-        public ResourceId buf;
 
+        public ResourceId buf;
         public UInt32 offset;
         public UInt32 stride;
 
@@ -376,6 +377,7 @@ namespace renderdoc
         public bool showAlpha;
 
         public PrimitiveTopology topo;
+        public UInt32 numVerts;
 
         public bool unproject;
         public float nearPlane;