[AD] Emscripten port: writing persistent files

[Max Savenkov <max.savenkov@xxxxxxxxxx>]

By default, Emscripten provides implementation for various file and
filesystem-related functions which reads data from memory, using
embedded or preloaded packed filesystems. This allows programs to read
data as usual with fopen etc.

However, if you WRITE any files, they are not really stored anywhere
and will be lost when page is reloaded. To avoid this, Emscripten
provides API to write to IndexedDB, which allows persistent storage of
files for web pages.

To allow user to work with both in-memory and with IDB data, I think I
will need to write a file interface, like one for PhysFS, which will
user will have to set when he is working with persistent data. So, if
I want to load a resource, then save and load game state, the
pseudo-code would look like this:

// Load resource from memory
load_resource( "res/res1.dat" );

#ifdef EMSCRIPTEN
// Create and mount directory 'saves' (as per
https://github.com/kripken/emscripten/blob/master/tests/fs/test_idbfs_sync.c
example)
al_ems_mount_idbfs( "saves/" );

// Set IDBFS file interface
set_file_interface( al_ems_get_file_interface() );
#endif

// Save file to IDBFS
write_save( "saves/save1.sav" );

#ifdef EMSCRIPTEN
// Restore default file interface for further loading of resources etc.
set_file_interface( al_get_default_file_interface() );
#endif

This will require programs that used just to write things anywhere to
be modified with ugly #ifdefs, but I can see no way to do this
transparently. So, unless anyone has a better idea, I will do it this
way.