working around drawImage issue on Android 2.1 devices. r=erilem (closes #3243)

git-svn-id: http://svn.openlayers.org/trunk/openlayers@11868 dc9f47b5-9b13-0410-9fdd-eb0c1a62fdaf

lib/OpenLayers/Renderer/Canvas.js

@@ -224,8 +224,15 @@ OpenLayers.Renderer.Canvas = OpenLayers.Class(OpenLayers.Renderer, {
             // TODO: check that we haven't moved
             var canvas = this.canvas;
             canvas.globalAlpha = opacity;
+            var factor = OpenLayers.Renderer.Canvas.drawImageScaleFactor ||
+                (OpenLayers.Renderer.Canvas.drawImageScaleFactor =
+                    /android 2.1/.test(navigator.userAgent.toLowerCase()) ?
+                        // 320 is the screen width of the G1 phone, for which
+                        // drawImage works out of the box.
+                        320 / window.screen.width : 1
+                );
             canvas.drawImage(
-                img, x, y, width, height
+                img, x*factor, y*factor, width*factor, height*factor
             );
             if (this.hitDetection) {
                 this.setHitContextStyle("fill", featureId);
@@ -680,3 +687,11 @@ OpenLayers.Renderer.Canvas.LABEL_FACTOR = {
     "t": 0,
     "b": -1
 };
+
+/**
+ * Constant: OpenLayers.Renderer.Canvas.drawImageScaleFactor
+ * {Number} Scale factor to apply to the canvas drawImage arguments. This
+ *     is always 1 except for Android 2.1 devices, to work around
+ *     http://code.google.com/p/android/issues/detail?id=5141.
+ */
+OpenLayers.Renderer.Canvas.drawImageScaleFactor = null;