CDROM: Revamp/improve preload image procedure

src/core/cdrom_async_reader.h

@@ -6,6 +6,8 @@
 #include <condition_variable>
 #include <thread>
 
+class ProgressCallback;
+
 class CDROMAsyncReader
 {
 public:
@@ -40,6 +42,9 @@ public:
   void SetMedia(std::unique_ptr<CDImage> media);
   std::unique_ptr<CDImage> RemoveMedia();
 
+  /// Precaches image, either to memory, or using the underlying image precache.
+  bool Precache(ProgressCallback* callback);
+
   void QueueReadSector(CDImage::LBA lba);
 
   bool WaitForReadToComplete();