ALL: Load savegame thumbnail only when necessary

This commit introduces the following changes:

1. Graphics::loadThumbnail()

   Now returns a boolean and takes a new argument skipThumbnail which
   defaults to false. In case of true, loadThumbnail() reads past the
   thumbnail data in the input stream instead of actually loading the
   thumbnail. This simplifies savegame handling where, up until now,
   many engines always read the whole savegame metadata (including
   the thumbnail) and then threw away the thumbnail when not needed
   (which is in almost all cases, the most common exception being
   MetaEngine::querySaveMetaInfos() which is responsible for loading
   savegame metadata for displaying it in the GUI launcher.

2. readSavegameHeader()

   Engines which already implement such a method (name varies) now take
   a new argument skipThumbnail (default: true) which is passed
   through to loadThumbnail(). This means that the default case for
   readSavegameHeader() is now _not_ loading the thumbnail from a
   savegame and just reading past it. In those cases, e.g.
   querySaveMetaInfos(), where we actually are interested in loading
   the thumbnail readSavegameHeader() needs to explicitely be called
   with skipThumbnail == false.

   Engines whose readSavegameHeader() (name varies) already takes an
   argument loadThumbnail have been adapted to have a similar
   prototype and semantics.
   I.e. readSaveHeader(in, loadThumbnail, header) now is
   readSaveHeader(in, header, skipThumbnail).

3. Error handling

   Engines which previously did not check the return value of
   readSavegameHeader() (name varies) now do so ensuring that possibly
   broken savegames (be it a broken thumbnail or something else) don't
   make it into the GUI launcher list in the first place.

2 files changed, 11 insertions, 4 deletions

diff --git a/engines/dm/dm.h b/engines/dm/dm.h
index 16307778ba..e330cb6be7 100644
--- a/engines/dm/dm.h
+++ b/engines/dm/dm.h
@@ -332,7 +332,7 @@ public:
 	Thing _thingParty;				 // @ C0xFFFF_THING_PARTY
 };
 
-bool readSaveGameHeader(Common::InSaveFile *in, SaveGameHeader *header);
+WARN_UNUSED_RESULT bool readSaveGameHeader(Common::InSaveFile *in, SaveGameHeader *header, bool skipThumbnail = true);
 
 } // End of namespace DM
 
diff --git a/engines/dm/loadsave.cpp b/engines/dm/loadsave.cpp
index 3d8f76c33f..864a726367 100644
--- a/engines/dm/loadsave.cpp
+++ b/engines/dm/loadsave.cpp
@@ -70,7 +70,10 @@ LoadgameResult DMEngine::loadgame(int16 slot) {
 		file = saveFileManager->openForLoading(fileName);
 
 		SaveGameHeader header;
-		readSaveGameHeader(file, &header);
+		if (!readSaveGameHeader(file, &header)) {
+			delete file;
+			return kDMLoadgameFailure;
+		}
 
 		warning("MISSING CODE: missing check for matching format and platform in save in f435_loadgame");
 
@@ -397,7 +400,7 @@ bool DMEngine::writeCompleteSaveFile(int16 saveSlot, Common::String& saveDescrip
 	return true;
 }
 
-bool readSaveGameHeader(Common::InSaveFile *in, SaveGameHeader *header) {
+WARN_UNUSED_RESULT bool readSaveGameHeader(Common::InSaveFile *in, SaveGameHeader *header, bool skipThumbnail) {
 	uint32 id = in->readUint32BE();
 
 	// Check if it's a valid ScummVM savegame
@@ -419,7 +422,11 @@ bool readSaveGameHeader(Common::InSaveFile *in, SaveGameHeader *header) {
 	header->_descr.setDescription(saveName);
 
 	// Get the thumbnail
-	header->_descr.setThumbnail(Graphics::loadThumbnail(*in));
+	Graphics::Surface *thumbnail;
+	if (!Graphics::loadThumbnail(*in, thumbnail, skipThumbnail)) {
+		return false;
+	}
+	header->_descr.setThumbnail(thumbnail);
 
 	uint32 saveDate = in->readUint32BE();
 	uint16 saveTime = in->readUint16BE();