Add pdfBackground constructor property (#4207)

src/common.h

@@ -74,6 +74,7 @@ namespace sharp {
     int textSpacing;
     VipsTextWrap textWrap;
     int textAutofitDpi;
+    std::vector<double> pdfBackground;
 
     InputDescriptor():
       buffer(nullptr),
@@ -108,7 +109,8 @@ namespace sharp {
       textRgba(false),
       textSpacing(0),
       textWrap(VIPS_TEXT_WRAP_WORD),
-      textAutofitDpi(0) {}
+      textAutofitDpi(0),
+      pdfBackground{ 255.0, 255.0, 255.0, 255.0 } {}
   };
 
   // Convenience methods to access the attributes of a Napi::Object