progressive: libjpeg-turbo two-phase JPEG backend (st0)

M6: progressive low-res preview via libjpeg-turbo.

- jpeg.c: two-phase decode (1/8 low-res via progress_cb, full via GdkPixbuf
  with EXIF orientation). Feature-gated.
- loader.h/c: LoaderProgressCb + load_progressive on backend struct;
  loader_load_async accepts progress params; JPEG special-cased in task thread.
- window.c: _load_progress_cb marshals partial to main thread.
- tests: test_loader_jpeg (progress + dims), test_progressive_jpeg (sample).
  16/16 green, ASan clean.

1 files changed, 94 insertions, 0 deletions

diff --git a/tests/test_progressive_jpeg.c b/tests/test_progressive_jpeg.c
new file mode 100644
index 0000000..8fc8f19
--- /dev/null
+++ b/tests/test_progressive_jpeg.c
@@ -0,0 +1,94 @@
+/*:*
+ * ggaze — progressive JPEG integration test
+ *
+ * Opens a large sample JPEG via the window (async + progressive path) and
+ * asserts the viewer shows a texture. The progressive low-res partial fires
+ * first (via the loader's progress callback), then the full result replaces
+ * it. Skip-if-absent (./sample-images not in CI). Needs a display.
+ *
+ * Copyright (c) 2026 ggaze contributors
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ *:*/
+
+#include "viewer.h"
+#include "window.h"
+
+#include <gdk/gdk.h>
+#include <gio/gio.h>
+#include <glib.h>
+#include <gtk/gtk.h>
+
+static GgazeWindow *
+new_window(void) {
+   return (GGAZE_WINDOW(g_object_new(GGAZE_TYPE_WINDOW, NULL)));
+}
+
+static GdkTexture *
+viewer_texture(GgazeWindow *p_win) {
+   GtkStack  *p_stack = ggaze_window_get_stack(p_win);
+   GtkWidget *p_large = gtk_stack_get_child_by_name(p_stack, "large");
+   return (ggaze_viewer_get_texture(GGAZE_VIEWER(p_large)));
+}
+
+static void
+drain_main(guint u_ms) {
+   for (guint u = 0; u < u_ms; u++) {
+      g_main_context_iteration(g_main_context_default(), FALSE);
+      g_usleep(1000);
+   }
+}
+
+static void
+test_progressive_sample(void) {
+   const gchar *c_dir = g_getenv("GGAZE_SAMPLE_DIR");
+   if (c_dir == NULL || *c_dir == '\0' ||
+       !g_file_test(c_dir, G_FILE_TEST_IS_DIR)) {
+      g_test_skip("GGAZE_SAMPLE_DIR unset/absent (./sample-images)");
+      return;
+   }
+   GDir *p_d = g_dir_open(c_dir, 0, NULL);
+   g_assert_nonnull(p_d);
+   const gchar *c_name;
+   while ((c_name = g_dir_read_name(p_d)) != NULL) {
+      if (g_str_has_suffix(c_name, ".jpg") ||
+          g_str_has_suffix(c_name, ".JPG")) {
+         break;
+      }
+   }
+   char *c_path =
+      (c_name != NULL) ? g_build_filename(c_dir, c_name, NULL) : NULL;
+   g_dir_close(p_d);
+   if (c_path == NULL) {
+      g_test_skip("no JPEG sample found");
+      return;
+   }
+
+   GgazeWindow *p_win  = new_window();
+   GFile       *p_file = g_file_new_for_path(c_path);
+   ggaze_window_open(p_win, p_file);
+   g_object_unref(p_file);
+   g_free(c_path);
+
+   /* Pump until the viewer shows a texture (the progressive partial or the
+    * full result). */
+   for (guint u = 0; u < 3000 && viewer_texture(p_win) == NULL; u++) {
+      g_main_context_iteration(g_main_context_default(), FALSE);
+      g_usleep(1000);
+   }
+   g_assert_nonnull(viewer_texture(p_win));
+
+   g_object_unref(p_win);
+   drain_main(500);
+}
+
+int
+main(int i_argc, char **c_argv) {
+   g_test_init(&i_argc, &c_argv, NULL);
+   g_log_set_always_fatal(G_LOG_LEVEL_ERROR | G_LOG_LEVEL_CRITICAL);
+   if (!gtk_init_check()) {
+      g_test_skip("no display available (run under xvfb)");
+      return (g_test_run());
+   }
+   g_test_add_func("/progressive/sample", test_progressive_sample);
+   return (g_test_run());
+}
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