Merge pull request #354 from UnityTech/fix_raster_cache_assert2

fix assert when devicepixelratio is not integer

Runtime/ui/geometry.cs

                 Mathf.Ceil(this.right), Mathf.Ceil(this.bottom));
         }
 
-        public Rect roundOut(float devicePixelRatio) {
+        public Rect roundOutScale(float scale) {
+            return fromLTRB(
+                Mathf.Floor(this.left * scale), 
+                Mathf.Floor(this.top * scale),
+                Mathf.Ceil(this.right * scale), 
+                Mathf.Ceil(this.bottom * scale));
+        }
+        
+        public Rect withDevicePixelRatio(float devicePixelRatio) {
             return fromLTRB(
                 Mathf.Floor(this.left * devicePixelRatio) / devicePixelRatio, 
                 Mathf.Floor(this.top * devicePixelRatio) / devicePixelRatio,

Runtime/ui/renderer/compositeCanvas/flow/raster_cache.cs

         public readonly float devicePixelRatio;
 
         public void draw(Canvas canvas) {
-            var bounds = canvas.getTotalMatrix().mapRect(this.logicalRect).roundOut(this.devicePixelRatio);
+            var boundRect = canvas.getTotalMatrix().mapRect(this.logicalRect);
+            var bounds = boundRect.withDevicePixelRatio(this.devicePixelRatio);
-                var textureWidth = Mathf.CeilToInt(bounds.width * this.devicePixelRatio);
-                var textureHeight = Mathf.CeilToInt(bounds.height * this.devicePixelRatio);
+                var boundsInPixel = boundRect.roundOutScale(this.devicePixelRatio);
+                var textureWidth = Mathf.CeilToInt(boundsInPixel.width);
+                var textureHeight = Mathf.CeilToInt(boundsInPixel.height);
                 
                 //it is possible that there is a minor difference between the bound size and the image size (1 pixel at
                 //most) due to the roundOut operation when calculating the bounds if the elements in the canvas transform
 
         RasterCacheResult _rasterizePicture(Picture picture, Matrix3 transform, float devicePixelRatio,
             int antiAliasing, MeshPool meshPool) {
-            var bounds = transform.mapRect(picture.paintBounds).roundOut(devicePixelRatio);
+            var boundRect = transform.mapRect(picture.paintBounds);
+            var bounds = boundRect.withDevicePixelRatio(devicePixelRatio);
+            var boundsInPixel = boundRect.roundOutScale(devicePixelRatio);
-                Mathf.CeilToInt((bounds.width * devicePixelRatio)),
-                Mathf.CeilToInt((bounds.height * devicePixelRatio)),
+                Mathf.CeilToInt(boundsInPixel.width),
+                Mathf.CeilToInt(boundsInPixel.height),
                 RenderTextureFormat.Default, 24) {
                 useMipMap = false,
                 autoGenerateMips = false,