pdfviewer: set the default background to transparent (to match render_picture default), and fix a b…

experimental/PdfViewer/pdf_viewer_main.cpp

Index: experimental/PdfViewer/pdf_viewer_main.cpp
===================================================================
--- experimental/PdfViewer/pdf_viewer_main.cpp	(revision 10357)
+++ experimental/PdfViewer/pdf_viewer_main.cpp	(working copy)
@@ -139,7 +139,7 @@
                                                    PNG_FILE_EXTENSION);
 }
 
-static void setup_bitmap(SkBitmap* bitmap, int width, int height, SkColor color = SK_ColorWHITE) {
+static void setup_bitmap(SkBitmap* bitmap, int width, int height, SkColor color = SK_ColorTRANSPARENT) {
     bitmap->setConfig(SkBitmap::kARGB_8888_Config, width, height);
 
     bitmap->allocPixels();