Fix orphans check

This effectively forbids line breaks before the first line. Also, check
the orphans property on the block parent, as it only applies to block container
elements per spec.

Based on work in #1356
Fixes #1617

Co-authored-by: David Sickmiller <david@sickmiller.com>

src/FrameDecorator/Page.php

@@ -364,7 +364,12 @@ protected function _page_break_allowed(Frame $frame)
 
                 // Rule C
                 $block_parent = $frame->find_block_parent();
-                if (count($block_parent->get_line_boxes()) < $frame->get_style()->orphans) {
+                $parent_style = $block_parent->get_style();
+                // The line number of the frame can be less than the current
+                // number of line boxes, in case we are backtracking. As long as
+                // we are not checking for widows yet, just checking against the
+                // number of line boxes is sufficient in most cases, though.
+                if (count($block_parent->get_line_boxes()) <= $parent_style->orphans) {
                     Helpers::dompdf_debug("page-break", "orphans");
 
                     return false;