Updated the screen implementation according to the spec

web/esop-ext.js

@@ -79,6 +79,8 @@ function ESOPExtensions() {
     // also, we need to set up the screen vector
     var rAF = window.requestAnimationFrame || window.webkitRequestAnimationFrame
     rAF(function vecframe() {
+      //the order is: clear - vector - render
+      vm.setramblk(vramOffset, new Uint8Array(vramSize))
       vm.triggerVector(0)
       renderScreen()
       if(vm.active)

web/uxncore.js

@@ -171,6 +171,7 @@ function UxnCore() {
     getdev: function(port) {return mem.dev[port]},
     getram: function(start, length) {return mem.ram.subarray(start, start+length)},
     setram: function(addr, bVal) {mem.ram[addr] = bVal},
+    setramblk: function(addr, blk) {mem.ram.set(blk, addr)},
     setWriteHook: function(port, cb) {
       if(!cb) cb = function(){}
       devWriteHooks[port] = cb