Skip any whitespace after the first object in linearized PDFs (issue 17665)

This way the code is now consistent with the non-linearized branch in the `PDFDocument.startXRef` getter.

src/core/document.js

@@ -930,7 +930,14 @@ class PDFDocument {
       // Find the end of the first object.
       stream.reset();
       if (find(stream, ENDOBJ_SIGNATURE)) {
-        startXRef = stream.pos + 6 - stream.start;
+        stream.skip(6);
+
+        let ch = stream.peekByte();
+        while (isWhiteSpace(ch)) {
+          stream.pos++;
+          ch = stream.peekByte();
+        }
+        startXRef = stream.pos - stream.start;
       }
     } else {
       // Find `startxref` by checking backwards from the end of the file.

src/core/worker.js

@@ -880,6 +880,9 @@ class WorkerMessageHandler {
           .ensureXRef("trailer")
           .then(trailer => trailer.get("Prev"));
       });
+      handler.on("GetStartXRefPos", function (data) {
+        return pdfManager.ensureDoc("startXRef");
+      });
       handler.on("GetAnnotArray", function (data) {
         return pdfManager.getPage(data.pageIndex).then(function (page) {
           return page.annotations.map(a => a.toString());

src/display/api.js

@@ -768,6 +768,9 @@ class PDFDocumentProxy {
       Object.defineProperty(this, "getXRefPrevValue", {
         value: () => this._transport.getXRefPrevValue(),
       });
+      Object.defineProperty(this, "getStartXRefPos", {
+        value: () => this._transport.getStartXRefPos(),
+      });
       Object.defineProperty(this, "getAnnotArray", {
         value: pageIndex => this._transport.getAnnotArray(pageIndex),
       });
@@ -2349,6 +2352,10 @@ class WorkerTransport {
         value: () =>
           this.messageHandler.sendWithPromise("GetXRefPrevValue", null),
       });
+      Object.defineProperty(this, "getStartXRefPos", {
+        value: () =>
+          this.messageHandler.sendWithPromise("GetStartXRefPos", null),
+      });
       Object.defineProperty(this, "getAnnotArray", {
         value: pageIndex =>
           this.messageHandler.sendWithPromise("GetAnnotArray", { pageIndex }),