[DVD] Ignore buffer when seeking backwards

Unfortunately this fix re-breaks Arc Rise Fantasia, so we must restore its INI as well.

Data/Sys/GameSettings/RPJ.ini

@@ -2,6 +2,7 @@
 
 [Core]
 # Values set here will override the main Dolphin settings.
+FastDiscSpeed = True
 
 [EmuState]
 # The Emulation State. 1 is worst, 5 is best, 0 is not set.

Source/Core/Core/HW/DVDInterface.cpp

@@ -1301,7 +1301,11 @@ u64 SimulateDiscReadTime(u64 offset, u32 length)
 	u64 disk_read_duration = CalculateRawDiscReadTime(offset, length) +
 		SystemTimers::GetTicksPerSecond() / 1000 * DISC_ACCESS_TIME_MS;
 
-	if (offset + length > s_last_read_offset + 1024 * 1024)
+	// Assume unbuffered read if the read we are performing asks for data >
+	// 1MB past the end of the last read *or* asks for data before the last
+	// read. It assumes the buffer is only used when reading small amounts
+	// forward.
+	if (offset + length > s_last_read_offset + 1024 * 1024 || offset < s_last_read_offset)
 	{
 		// No buffer; just use the simple seek time + read time.
 		DEBUG_LOG(DVDINTERFACE, "Seeking %" PRId64 " bytes",