Centralize fullscreenElement usage. NFC (#24708)

I will likely followup with another PR to reduce the size of this
function using `MIN_XXX_VERSION` macros, but I wanted to keep this
change simple and NFC.

Author: sbc100

src/lib/libbrowser.js

@@ -8,6 +8,7 @@
 var LibraryBrowser = {
   $Browser__deps: [
     '$callUserCallback',
+    '$getFullscreenElement',
     '$safeSetTimeout',
     '$warnOnce',
 #if FILESYSTEM
@@ -232,9 +233,7 @@ var LibraryBrowser = {
       function fullscreenChange() {
         Browser.isFullscreen = false;
         var canvasContainer = canvas.parentNode;
-        if ((document['fullscreenElement'] || document['mozFullScreenElement'] ||
-             document['msFullscreenElement'] || document['webkitFullscreenElement'] ||
-             document['webkitCurrentFullScreenElement']) === canvasContainer) {
+        if (getFullscreenElement() === canvasContainer) {
           canvas.exitFullscreen = Browser.exitFullscreen;
           if (Browser.lockPointer) canvas.requestPointerLock();
           Browser.isFullscreen = true;
@@ -533,9 +532,7 @@ var LibraryBrowser = {
           h = Math.round(w / Module['forcedAspectRatio']);
         }
       }
-      if (((document['fullscreenElement'] || document['mozFullScreenElement'] ||
-           document['msFullscreenElement'] || document['webkitFullscreenElement'] ||
-           document['webkitCurrentFullScreenElement']) === canvas.parentNode) && (typeof screen != 'undefined')) {
+      if ((getFullscreenElement() === canvas.parentNode) && (typeof screen != 'undefined')) {
          var factor = Math.min(screen.width / w, screen.height / h);
          w = Math.round(w * factor);
          h = Math.round(h * factor);

src/lib/libglfw.js

@@ -87,6 +87,7 @@ var LibraryGLFW = {
     'malloc', 'free',
     '$MainLoop',
     '$stringToNewUTF8',
+    '$getFullscreenElement',
     'emscripten_set_window_title',
 #if FILESYSTEM
     '$FS',
@@ -1168,9 +1169,7 @@ var LibraryGLFW = {
       function fullscreenChange() {
         Browser.isFullscreen = false;
         var canvasContainer = canvas.parentNode;
-        if ((document['fullscreenElement'] || document['mozFullScreenElement'] ||
-          document['msFullscreenElement'] || document['webkitFullscreenElement'] ||
-          document['webkitCurrentFullScreenElement']) === canvasContainer) {
+        if (getFullscreenElement() === canvasContainer) {
           canvas.exitFullscreen = Browser.exitFullscreen;
           if (Browser.lockPointer) canvas.requestPointerLock();
           Browser.isFullscreen = true;
@@ -1239,9 +1238,7 @@ var LibraryGLFW = {
           h = Math.round(w / Module['forcedAspectRatio']);
         }
       }
-      if (((document['fullscreenElement'] || document['mozFullScreenElement'] ||
-        document['msFullscreenElement'] || document['webkitFullscreenElement'] ||
-        document['webkitCurrentFullScreenElement']) === canvas.parentNode) && (typeof screen != 'undefined')) {
+      if ((getFullscreenElement() === canvas.parentNode) && (typeof screen != 'undefined')) {
         var factor = Math.min(screen.width / w, screen.height / h);
         w = Math.round(w * factor);
         h = Math.round(h * factor);

src/lib/libhtml5.js

@@ -249,6 +249,13 @@ var LibraryHTML5 = {
     },
   },
 
+  $getFullscreenElement__internal: true,
+  $getFullscreenElement() {
+    return document.fullscreenElement || document.mozFullScreenElement ||
+           document.webkitFullscreenElement || document.webkitCurrentFullScreenElement ||
+           document.msFullscreenElement;
+  },
+
   $registerKeyEventCallback__noleakcheck: true,
   $registerKeyEventCallback__deps: ['$JSEvents', '$findEventTarget', '$stringToUTF8', 'malloc'],
   $registerKeyEventCallback: (target, userData, useCapture, callbackfunc, eventTypeId, eventTypeString, targetThread) => {
@@ -990,9 +997,9 @@ var LibraryHTML5 = {
     return {{{ cDefs.EMSCRIPTEN_RESULT_SUCCESS }}};
   },
 
-  $fillFullscreenChangeEventData__deps: ['$JSEvents', '$stringToUTF8'],
+  $fillFullscreenChangeEventData__deps: ['$JSEvents', '$stringToUTF8', '$getFullscreenElement'],
   $fillFullscreenChangeEventData: (eventStruct) => {
-    var fullscreenElement = document.fullscreenElement || document.mozFullScreenElement || document.webkitFullscreenElement || document.msFullscreenElement;
+    var fullscreenElement = getFullscreenElement();
     var isFullscreen = !!fullscreenElement;
 #if !SAFE_HEAP
     // Assigning a boolean to HEAP32 with expected type coercion.
@@ -1204,15 +1211,7 @@ var LibraryHTML5 = {
     var oldImageRendering = canvas.style.imageRendering;
 
     function restoreOldStyle() {
-      var fullscreenElement = document.fullscreenElement
-#if MIN_FIREFOX_VERSION <= 63 // https://caniuse.com/#feat=mdn-api_documentorshadowroot_fullscreenelement
-        || document.mozFullScreenElement
-#endif
-#if MIN_CHROME_VERSION != TARGET_NOT_SUPPORTED || MIN_SAFARI_VERSION != TARGET_NOT_SUPPORTED // https://caniuse.com/#feat=mdn-api_documentorshadowroot_fullscreenelement
-        || document.webkitFullscreenElement
-#endif
-        ;
-      if (!fullscreenElement) {
+      if (!getFullscreenElement()) {
         document.removeEventListener('fullscreenchange', restoreOldStyle);
 
 #if MIN_FIREFOX_VERSION <= 63 // https://caniuse.com/#feat=mdn-api_element_fullscreenchange_event