Canvas test: make sure canvas is ready

before:
Totals: 80 passed, 653 failed, 0 skipped, 0 blacklisted, 35852ms
after:
Totals: 729 passed, 4 failed, 0 skipped, 0 blacklisted, 29599ms

The last four failures are due to high dpi issues.

Change-Id: Ie937091558395572dc281ee5a5b90848ea9d2081
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>

tests/auto/quick/qquickcanvasitem/data/CanvasTestCase.qml

@@ -31,7 +31,9 @@ TestCase {
   }
 
   function createCanvasObject(data) {
-    return component.createObject(testCase, data.properties);
+      var canvas = component.createObject(testCase, data.properties);
+      waitForRendering(canvas);
+      return canvas;
   }
 
   function comparePixel(ctx,x,y,r,g,b,a, d)

tests/auto/quick/qquickcanvasitem/data/tst_canvas.qml

@@ -590,6 +590,7 @@ CanvasTestCase {
         verify(canvas);
         canvas.width = 100;
         canvas.height = 100;
+        waitForRendering(canvas);
 
         var ctx = canvas.getContext("2d");
 

tests/auto/quick/qquickcanvasitem/data/tst_image.qml

@@ -667,6 +667,7 @@ CanvasTestCase {
        var canvas2 = Qt.createQmlObject("import QtQuick 2.0; Canvas{renderTarget:Canvas.Image; renderStrategy:Canvas.Immediate}", canvas);
        canvas2.width = 100;
        canvas2.height = 50;
+       waitForRendering(canvas2);
        var ctx2 = canvas2.getContext('2d');
        ctx2.fillStyle = '#0f0';
        ctx2.fillRect(0, 0, 100, 50);