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js: load games using FileReader.readAsArrayBuffer()

Using .readAsText() meant that trying to load a non-text file (for
instance something that's not a save file at all) would generate an
"RuntimeError: index out of bounds".  This would then leave the
Emscripten runtime in a broken state.

It might even be possible for a real save file not to be valid UTF-8,
for instance if it came from a platform that used a different character
encoding for random seeds.

There's still a problem with opening very large files, apparently
because Emscripten tries to stuff the entire file onto the C stack.
That will probably have to be fixed by properly exposing the incremental
file-loading API to JavaScript.

--- a/emccpre.js

+++ b/emccpre.js

@@ -423,7 +423,7 @@

     // 'number' is used for C pointers

     var get_save_file = Module.cwrap('get_save_file', 'number', []);

     var free_save_file = Module.cwrap('free_save_file', 'void', ['number']);

-    var load_game = Module.cwrap('load_game', 'void', ['string', 'number']);

+    var load_game = Module.cwrap('load_game', 'void', ['array', 'number']);

     if (save_button) save_button.onclick = function(event) {

         if (dlg_dimmer === null) {

@@ -457,10 +457,10 @@

                     var file = input.files.item(0);

                     var reader = new FileReader();

                     reader.addEventListener("loadend", function() {

-                        var string = reader.result;

-                        load_game(string, string.length);

+                        var array = new Uint8Array(reader.result);

+                        load_game(array, array.length);

                     });

-                    reader.readAsText(file);

+                    reader.readAsArrayBuffer(file);

                 }

             });

             input.click();