add event listeners to canvas

it's stupid to capture them in the whole document just to not have
ref to display

lib/gamejs/event.js

@@ -273,16 +273,16 @@ exports.init = function() {
 
    // IEFIX does not support addEventListener on document itself
    // MOZFIX but in moz & opera events don't reach body if mouse outside window or on menubar
-   // hook onto document.body not canvas to avoid dependancy into gamejs.display
-   document.addEventListener('mousedown', onMouseDown, false);
-   document.addEventListener('mouseup', onMouseUp, false);
-   document.addEventListener('keydown', onKeyDown, false);
-   document.addEventListener('keyup', onKeyUp, false);
-   document.addEventListener('mousemove', onMouseMove, false);
-   document.addEventListener('mousewheel', onMouseScroll, false);
+   var canvas = display.getSurface()._canvas;
+   canvas.addEventListener('mousedown', onMouseDown, false);
+   canvas.addEventListener('mouseup', onMouseUp, false);
+   canvas.addEventListener('keydown', onKeyDown, false);
+   canvas.addEventListener('keyup', onKeyUp, false);
+   canvas.addEventListener('mousemove', onMouseMove, false);
+   canvas.addEventListener('mousewheel', onMouseScroll, false);
    // MOZFIX
    // https://developer.mozilla.org/en/Code_snippets/Miscellaneous#Detecting_mouse_wheel_events
-   document.addEventListener('DOMMouseScroll', onMouseScroll, false);
-   document.addEventListener('beforeunload', onBeforeUnload, false);
+   canvas.addEventListener('DOMMouseScroll', onMouseScroll, false);
+   canvas.addEventListener('beforeunload', onBeforeUnload, false);
 
 };