Add an assertion when using old-school sprite loading with compressed sprites

[rasky]

Some examples on trunk still use the old-school sprite loading instead of sprite_load:

    fp = dfs_open("/background1.sprite");
    sprite_t *background1 = malloc( dfs_size( fp ) );
    dfs_read( background1, 1, dfs_size( fp ), fp );
    dfs_close( fp );

This is compatible with sprites created with mksprite with the old syntax. When switching to the new syntax, mksprite compresses sprites by default so they're not compatible anymore. Instead, users get random crashes.

We could try and add an assertion here, maybe in __sprite_upgrade().

[rasky]

Fixed in preview: c1a13d9