"Fix "t->c->value.argSize == func->parmTotal" Assertion in Scripts, #303"
had broken old savegames because the script checksum
(idProgram::CalculateChecksum()) changed, see #344.
This is fixed now, also the BUILD_NUMBER is increased so old savegames
can be identified for a workaround.

Don't use this commit without the "Add information about dhewm3 build
 to savegames" commit, which further modifies the savegame format
(for the new BUILD_NUMBER 1305). In dhewm3-sdk this commit is directly
before that commit, but unfortunately I fucked up when cherry-picking
the "newer" commits onto librecoop, missing a few of them, so this one
and the previous one and the next three commits are out of order

in dhewm3-sdk this commit was called "From dhewm3: Add absolute mouse
 mode and refactor mouse grabbing code", but the sys.h changes are
already merged

The change in Stub_SDL_endian.h is just me being paranoid.
(side-note: it *might* make sense to replace all that inline-asm in
 Stub_SDL_endian.h with compiler intrinsics like GCC/clangs
 __builtin_bswap* and MSVCs _byteswap_* - if they are supported in
 all relevant compiler versions. GCC supports 32 and 64 bit swaps since
 4.3, 16bit since 4.8 and 128bit since 11.x; clang supports 32 and
 64 bit swaps at least since 3.0 and 16bit swaps since 3.2;
 VS introduced _byteswap_ushort, .._ulong and .._uint64 in VS2003)

Some level scripts in d3xp (erebus4, erebus4, phobos2) use
  $entity.startSoundShader( "", SND_CHANNEL_BODY );
(or whatever channel) to stop the sound currently playing there.
With s_playDefaultSound 1 that results in a beep..
Added a special case to that event implementation to call StopSound()
instead when the soundName is "" (or NULL)