Use main buffers for utility draws

Source/Core/VideoCommon/IndexGenerator.cpp

@@ -3,6 +3,7 @@
 // Refer to the license.txt file included.
 
 #include <cstddef>
+#include <cstring>
 
 #include "Common/CommonTypes.h"
 #include "Common/Compiler.h"
@@ -56,6 +57,13 @@ void IndexGenerator::AddIndices(int primitive, u32 numVerts)
   base_index += numVerts;
 }
 
+void IndexGenerator::AddExternalIndices(const u16* indices, u32 num_indices, u32 num_vertices)
+{
+  std::memcpy(index_buffer_current, indices, sizeof(u16) * num_indices);
+  index_buffer_current += num_indices;
+  base_index += num_vertices;
+}
+
 // Triangles
 template <bool pr>
 DOLPHIN_FORCE_INLINE u16* IndexGenerator::WriteTriangle(u16* Iptr, u32 index1, u32 index2,