DVDInterface: Don't evict cache block i unless block i + 2 was read

Intends to fix https://bugs.dolphin-emu.org/issues/12279.
I have hardware tested the behavior, but I haven't tested the game.

Source/Core/Core/HW/DVD/DVDInterface.cpp

@@ -1512,27 +1512,38 @@ static void ScheduleReads(u64 offset, u32 length, const DiscIO::Partition& parti
     dvd_offset += DVD_ECC_BLOCK_SIZE;
   } while (length > 0);
 
-  // Update the buffer based on this read. Based on experimental testing,
+  // Evict blocks from the buffer which are unlikely to be used again after this read,
-  // we will only reuse the old buffer while reading forward. Note that the
+  // so that the buffer gets space for prefetching new blocks. Based on hardware testing,
-  // buffer start we calculate here is not the actual start of the buffer -
+  // the blocks which are kept are the most recently accessed block, the block immediately
-  // it is just the start of the portion we need to read.
+  // before it, and all blocks after it.
+  //
+  // If the block immediately before the most recently accessed block is not kept, loading
+  // screens in Pitfall: The Lost Expedition are longer than they should be.
+  // https://bugs.dolphin-emu.org/issues/12279
   const u64 last_block = dvd_offset;
-  if (last_block == buffer_start + DVD_ECC_BLOCK_SIZE && buffer_start != buffer_end)
+  constexpr u32 BUFFER_BACKWARD_SEEK_LIMIT = DVD_ECC_BLOCK_SIZE * 2;
+  if (last_block - buffer_start <= BUFFER_BACKWARD_SEEK_LIMIT && buffer_start != buffer_end)
   {
-    // Special case: reading less than one block at the start of the
+    // Special case: reading the first two blocks of the buffer doesn't change the buffer state
-    // buffer won't change the buffer state
   }
   else
   {
+    // Note that the s_read_buffer_start_offset value we calculate here is not the
+    // actual start of the buffer - it is just the start of the portion we need to read.
+    // The actual start of the buffer is s_read_buffer_end_offset - STREAMING_BUFFER_SIZE.
     if (last_block >= buffer_end)
+    {
       // Full buffer read
       s_read_buffer_start_offset = last_block;
+    }
     else
+    {
       // Partial buffer read
       s_read_buffer_start_offset = buffer_end;
+    }
 
-    s_read_buffer_end_offset = last_block + STREAMING_BUFFER_SIZE - DVD_ECC_BLOCK_SIZE;
+    s_read_buffer_end_offset = last_block + STREAMING_BUFFER_SIZE - BUFFER_BACKWARD_SEEK_LIMIT;
-    // Assume the buffer starts reading right after the end of the last operation
+    // Assume the buffer starts prefetching new blocks right after the end of the last operation
     s_read_buffer_start_time = current_time + ticks_until_completion;
     s_read_buffer_end_time =
         s_read_buffer_start_time +