GUACAMOLE-377: Do NOT combine display operations vertically unless ed…

…ges match exactly.

Doing otherwise can result in display operations overlapping. This is
because combining two vertically adjacent operations that have different
widths causes additional cells to be included that are not covered by
either original operation. If other operations are within those
additional cells, then the operations flushed for the frame will
overlap, consuming unnecessary additional processing and bandwidth.

src/libguac/display-plan-combine.c

@@ -311,6 +311,7 @@ void PFW_guac_display_plan_combine_vertically(guac_display_plan* plan) {
 
                     /* Combine adjacent updates if doing so is advantageous */
                     if (previous->related_op != NULL && cell->related_op != NULL
+                            && guac_display_plan_has_common_edge(previous->related_op, cell->related_op)
                             && guac_display_plan_combine_if_improved(previous->related_op, cell->related_op)) {
                         cell->related_op = previous->related_op;
                     }