LibPDF: Make Reference store two u32s instead of one

Reference used to be clever and stored the index of a ref in 18 bits
and the generation in 14 bits, so that both fit into a single u32.

However:
- It set MAX_REF_INDEX incorrectly (the max value of an 18-bit number
  is `(1 << 18) - 1`, not `(1 << 19) - 1`
- pdf_reference_1-7.pdf has 349223 objects, and that's larger
  than `(1 << 18) - 1` (which is 262143)

Since a Reference is stored in Value which is a Variant that also
stores a pointer, the size of Value is already 64-bit. So just don't
be clever here.

Makes pdf_reference_1-7.pdf get a bit further during decryption.

Userland/Libraries/LibPDF/Reference.h

@@ -12,34 +12,26 @@
 namespace PDF {
 
 class Reference {
-    // We store refs as u32, with 18 bits for the index and 14 bits for the
-    // generation index. The generation index is stored in the higher bits.
-    // This may need to be rethought later, as the max generation index is
-    // 2^16 and the max for the object index is probably 2^32 (I don't know
-    // exactly)
-    static constexpr auto MAX_REF_INDEX = (1 << 19) - 1;            // 2 ^ 19 - 1
-    static constexpr auto MAX_REF_GENERATION_INDEX = (1 << 15) - 1; // 2 ^ 15 - 1
-
 public:
     Reference(u32 index, u32 generation_index)
+        : m_ref_index(index)
+        , m_generation_index(generation_index)
     {
-        VERIFY(index < MAX_REF_INDEX);
-        VERIFY(generation_index < MAX_REF_GENERATION_INDEX);
-        m_combined = (generation_index << 14) | index;
     }
 
     [[nodiscard]] ALWAYS_INLINE u32 as_ref_index() const
     {
-        return m_combined & 0x3ffff;
+        return m_ref_index;
     }
 
     [[nodiscard]] ALWAYS_INLINE u32 as_ref_generation_index() const
     {
-        return m_combined >> 18;
+        return m_generation_index;
     }
 
 private:
-    u32 m_combined;
+    u32 m_ref_index;
+    u32 m_generation_index;
 };
 
 }