Fixes Trac#10619.
The thumbnail loading for the saved games was defaulting to
disabled so no thumbnail was being loaded and this caused trying
to show the thumbnail to crash scummvm.
So I have set the thumbnail pointer to be null if the thumbnail
doesn't get loaded and I've set the thumnail to not be skipped
so that thumbnails get shown when using the gui.
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.
The two different save/load dialogs from the original have been dropped
and replaced with the ScummVM save/load dialogs. The whole save/load
code has been rewritten to be endian safe and use our common code. The
original save format has not been preserved. The current implementation
crashes when loading, but it's a good start
