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/IndexGenerator.h

@@ -7,26 +7,29 @@
 
 #pragma once
 
+#include <array>
 #include "Common/CommonTypes.h"
 
 class IndexGenerator
 {
 public:
-  // Init
-  static void Init();
-  static void Start(u16* Indexptr);
+  void Init();
+  void Start(u16* index_ptr);
 
-  static void AddIndices(int primitive, u32 numVertices);
+  void AddIndices(int primitive, u32 num_vertices);
 
-  static void AddExternalIndices(const u16* indices, u32 num_indices, u32 num_vertices);
+  void AddExternalIndices(const u16* indices, u32 num_indices, u32 num_vertices);
 
   // returns numprimitives
-  static u32 GetNumVerts() { return base_index; }
-  static u32 GetIndexLen() { return (u32)(index_buffer_current - BASEIptr); }
-  static u32 GetRemainingIndices();
+  u32 GetNumVerts() const { return m_base_index; }
+  u32 GetIndexLen() const { return static_cast<u32>(m_index_buffer_current - m_base_index_ptr); }
+  u32 GetRemainingIndices() const;
 
 private:
-  static u16* index_buffer_current;
-  static u16* BASEIptr;
-  static u32 base_index;
+  u16* m_index_buffer_current = nullptr;
+  u16* m_base_index_ptr = nullptr;
+  u32 m_base_index = 0;
+
+  using PrimitiveFunction = u16* (*)(u16*, u32, u32);
+  std::array<PrimitiveFunction, 8> m_primitive_table{};
 };