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js: Make update_pixel_ratio more backward-compatible

Despite my stylistic downgrades, it still used two features not present
in Firefox 48, and hence KaiOS 2.5: passing options to addEventListener,
and calling addEventListener on a MediaQueryList at all.  Now it uses
the older addListener method and explicitly removes each listener as
soon as it's called.

--- a/emccpre.js

+++ b/emccpre.js

@@ -527,13 +527,16 @@

     /*

      * Arrange to detect changes of device pixel ratio.  Adapted from

      * <https://developer.mozilla.org/en-US/docs/Web/API/Window/

-     * devicePixelRatio> (CC0).

+     * devicePixelRatio> (CC0) to work on older browsers.

      */

+    var mql = null;

     var update_pixel_ratio = function() {

         var dpr = window.devicePixelRatio;

+        if (mql !== null)

+            mql.removeListener(update_pixel_ratio);

         resizable_div.style.width = onscreen_canvas.width / dpr + "px";

-        matchMedia(`(resolution: ${dpr}dppx)`)

-            .addEventListener("change", update_pixel_ratio, { once: true })

+        mql = window.matchMedia(`(resolution: ${dpr}dppx)`);

+        mql.addListener(update_pixel_ratio);

     }

     update_pixel_ratio();