VideoCommon/IndexGenerator: Eliminate static state

Now that we've extracted all of the stateless functions that can be
hidden, it's time to make the index generator a regular class with
active data members.

This can just be a member that sits within the vertex manager base
class. By deglobalizing the state of the index generator we also get rid
of the wonky dual-initializing that was going on within the OpenGL
backend.

Since the renderer is always initialized before the vertex manager, we
now only call Init() once throughout the execution lifecycle.

Source/Core/VideoCommon/VertexLoaderManager.cpp

@@ -283,8 +283,7 @@ int RunVertices(int vtx_attr_group, int primitive, int count, DataReader src, bo
 
   count = loader->RunVertices(src, dst, count);
 
-  IndexGenerator::AddIndices(primitive, count);
-
+  g_vertex_manager->AddIndices(primitive, count);
   g_vertex_manager->FlushData(count, loader->m_native_vtx_decl.stride);
 
   ADDSTAT(g_stats.this_frame.num_prims, count);