Fix the position of the thumbnails on the last line (bug 2016437)

The original issue can only be reproduce when the thumbnails are on two lines.
The fix is just a matter of computing the number of elements on the last line once we've
finished the first line.

Author: calixteman

web/pdf_thumbnail_viewer.js

@@ -898,11 +898,6 @@ class PDFThumbnailViewer {
         firstRightX ??= prevX + w;
         positionsX.push(prevX);
       }
-      if (reminder > 0 && i >= ii - reminder) {
-        const cx = x + w / 2;
-        positionsLastX.push(cx);
-        lastRightX ??= cx + w;
-      }
       if (y > prevY) {
         if (reminder === -1 && positionsX.length > 1) {
           reminder = ii % positionsX.length;
@@ -911,6 +906,11 @@ class PDFThumbnailViewer {
         firstBottomY ??= prevY + h;
         positionsY.push(prevY);
       }
+      if (reminder > 0 && i >= ii - reminder) {
+        const cx = x + w / 2;
+        positionsLastX.push(cx);
+        lastRightX ??= cx + w;
+      }
     }
     const space =
       positionsX.length > 1