SharedGraphics: Implement a simple PNG decoder.

This is extremely unoptimized, but it does successfully load "folder32.png"
so it must be at least somewhat correct. :^)

Userland/qs.cpp

@@ -0,0 +1,44 @@
+#include <SharedGraphics/PNGLoader.h>
+#include <LibGUI/GApplication.h>
+#include <LibGUI/GWindow.h>
+#include <LibGUI/GLabel.h>
+#include <stdio.h>
+
+int main(int argc, char** argv)
+{
+    GApplication app(argc, argv);
+
+#if 0
+    if (argc != 2) {
+        printf("usage: qs <image-file>\n");
+        return 0;
+    }
+#endif
+
+    const char* path = "/res/icons/folder32.png";
+    if (argc > 1)
+        path = argv[1];
+
+    auto bitmap = load_png(path);
+    if (!bitmap) {
+        fprintf(stderr, "Failed to load %s\n", path);
+        return 1;
+    }
+
+    auto* window = new GWindow;
+    window->set_title(String::format("qs: %s", path));
+    window->set_rect(200, 200, bitmap->width(), bitmap->height());
+
+    auto* widget = new GWidget;
+    widget->set_fill_with_background_color(true);
+    window->set_main_widget(widget);
+
+    auto* label = new GLabel(widget);
+    label->set_relative_rect({ 0, 0, bitmap->width(), bitmap->height() });
+    label->set_icon(move(bitmap));
+
+    window->set_should_exit_event_loop_on_close(true);
+    window->show();
+
+    return app.exec();
+}