gpu: Rewrite virtual memory manager using PageTable.

src/common/page_table.h

@@ -21,6 +21,8 @@ enum class PageType : u8 {
     RasterizerCachedMemory,
     /// Page is mapped to a I/O region. Writing and reading to this page is handled by functions.
     Special,
+    /// Page is allocated for use.
+    Allocated,
 };
 
 struct SpecialRegion {
@@ -66,7 +68,7 @@ struct PageTable {
      * Contains MMIO handlers that back memory regions whose entries in the `attribute` vector is
      * of type `Special`.
      */
-    boost::icl::interval_map<VAddr, std::set<SpecialRegion>> special_regions;
+    boost::icl::interval_map<u64, std::set<SpecialRegion>> special_regions;
 
     /**
      * Vector of fine grained page attributes. If it is set to any value other than `Memory`, then
@@ -74,6 +76,8 @@ struct PageTable {
      */
     std::vector<PageType> attributes;
 
+    std::vector<u64> backing_addr;
+
     const std::size_t page_size_in_bits{};
 };