PageTable: move backing addresses to a children class as the CPU page table does not need them.

This PR aims to reduce the memory usage in the CPU page table by moving
GPU specific parameters into a child class. This saves 1Gb of Memory for
most games.

src/common/page_table.h

@@ -76,9 +76,20 @@ struct PageTable {
      */
     std::vector<PageType> attributes;
 
-    std::vector<u64> backing_addr;
-
     const std::size_t page_size_in_bits{};
 };
 
+/**
+ * A more advanced Page Table with the ability to save a backing address when using it
+ * depends on another MMU.
+ */
+struct BackingPageTable : PageTable {
+    explicit BackingPageTable(std::size_t page_size_in_bits);
+    ~BackingPageTable();
+
+    void Resize(std::size_t address_space_width_in_bits);
+
+    std::vector<u64> backing_addr;
+};
+
 } // namespace Common