gui: previews: Fix incorrect pos calculation in cached previews

The cached order position introduced in 42e9ac5 is incorrect
as it does not consider the case when we are viewing any
other result page than the first.

Fix this by considering which page we are in when calculating
the offset

gui/mainwindow.cpp

@@ -940,12 +940,12 @@ void MainWindow::makePreviews(int page)
 			}
 		}
 	}
-	int end = previewsPerPage;
-	int begin = page * previewsPerPage - previewsPerPage;
-	if(begin < 0)
+	int length = previewsPerPage;
+	int beginOffset = page * previewsPerPage - previewsPerPage;
+	if(beginOffset < 0)
 	{
 		// Should not happen actually
-		begin = 0;
+		beginOffset = 0;
 	}
 
 	int currentScale = currentSelectedScale();
@@ -972,11 +972,14 @@ void MainWindow::makePreviews(int page)
 		renderTarget.path = sr.fileData.absPath;
 		renderTarget.page = (int)sr.page;
 		targets.append(renderTarget);
-		this->previewOrder[renderTarget.path + QString::number(renderTarget.page)] = previewPos++;
+
+		int pos = previewPos - beginOffset;
+		this->previewOrder[renderTarget.path + QString::number(renderTarget.page)] = pos;
+		++previewPos;
 	}
 	int numpages = ceil(static_cast<double>(targets.size()) / previewsPerPage);
 	ui->spinPreviewPage->setMaximum(numpages);
-	targets = targets.mid(begin, end);
+	targets = targets.mid(beginOffset, length);
 
 	ui->lblTotalPreviewPagesCount->setText(QString::number(numpages));
 	ui->previewProcessBar->setMaximum(targets.count());