Fix problem that page background color is not painted in the bleed area

Note: this fix is not very perfect and has limitation
- not fixed about background images
- when the background color has alpha value of rgba(), the page box (the area bleed box minus bleed area) background color becomes darker because both bleed box and page box have the same background color and it increases the opacity.

src/vivliostyle/page.js

@@ -433,6 +433,13 @@ vivliostyle.page.addPrinterMarks = (cascadedPageStyle, evaluatedPageSizeAndBleed
     const printerMarkOffset = adapt.css.toNumber(vivliostyle.page.defaultPrinterMarkOffset, context);
     const lineLength = adapt.css.toNumber(vivliostyle.page.defaultPrinterMarkLineLength, context);
 
+    if (bleed) {
+        const bgcolor = cascadedPageStyle["background-color"];
+        if (bgcolor && bgcolor.value) {
+            page.bleedBox.style.backgroundColor = bgcolor.value.stringValue()
+        }
+    }
+
     // corner marks
     if (crop) {
         Object.keys(vivliostyle.page.CornerMarkPosition).forEach(key => {