Fix a crash when changing presets in Inertia. Turns out that my

Javascript-side blitter creation function had forgotten to return the
new blitter's id, so the C code was still trying to use blitter #0
after it had been thrown away and replaced.

[originally from svn r9790]

emcclib.js

@@ -463,6 +463,7 @@ mergeInto(LibraryManager.library, {
         blitters[id] = document.createElement("canvas");
         blitters[id].width = w;
         blitters[id].height = h;
+        return id;
     },
 
     /*