Try getting backtrace from gdb if the test crashes

Run gdb on the core file, if one was generated.

Also remove any limits on core file size to try to ensure that it is
generated in the first place.

.github/workflows/ci.yml

@@ -232,7 +232,15 @@ jobs:
             fi
             export ASAN_OPTIONS="$ASAN_OPTIONS fast_unwind_on_malloc=0"
           fi
-          xvfb-run -a -s '-screen 0 1600x1200x24' ./test_gui
+          ulimit -c unlimited
+          /bin/bash -o pipefail -c 'xvfb-run -a -s "-screen 0 1600x1200x24" ./test_gui 2>&1 | tee test_gui.out' || rc=$?
+          if [ -n "$rc" ]; then
+            if fgrep -q '(core dumped)' test_gui.out; then
+                echo '*** Test crashed, trying to get more information ***'
+                gdb --quiet --core=core -ex 'where' -ex 'thread apply all bt' -ex 'q' ./test_gui
+            fi
+            exit $rc
+          fi
 
       - name: Building samples
         if: matrix.skip_testing != true && matrix.skip_gui != true && matrix.skip_samples != true