VideoCommon: merge triangle+list+point index buffers

We are used to render them out of order as long as everything else matches, but rendering order does matter, so we have to flush on primitive switch. This commit implements this flush.
Also as we flush on primitive switch, we don't have to create three different index buffers. All indices are now stored in one buffer.

This will slow down games which switch often primitive types (eg ztp), but it should be more accurate.

Source/Core/VideoBackends/D3D/VertexManager.h

@@ -32,11 +32,9 @@ private:
 	u32 m_vertex_buffer_cursor;
 	u32 m_vertex_draw_offset;
 	u32 m_index_buffer_cursor;
+	u32 m_index_draw_offset;
 	u32 m_current_vertex_buffer;
 	u32 m_current_index_buffer;
-	u32 m_triangle_draw_index;
-	u32 m_line_draw_index;
-	u32 m_point_draw_index;
 	typedef ID3D11Buffer* PID3D11Buffer;
 	PID3D11Buffer* m_index_buffers;
 	PID3D11Buffer* m_vertex_buffers;