LibDraw: Add ImageLoader, a simple abstraction for image loading

An ImageLoader is a generic interface for loading encoded image data of
any supported format. It has an ImageLoaderPlugin internally that does
all the work.

This patch adds an initial PNGImageLoaderPlugin that knows how to
retrieve the size of a PNG, and the bitmap. The API is divided into
size() and bitmap() to facilitate geometry-only decoding.
This will be useful in places like LibHTML where we need dimensions for
layout purposes but can wait with the bitmap until later.

Libraries/LibDraw/ImageLoader.h

@@ -0,0 +1,35 @@
+#pragma once
+
+#include <AK/NonnullRefPtr.h>
+#include <AK/OwnPtr.h>
+#include <AK/RefCounted.h>
+#include <LibDraw/Size.h>
+
+class GraphicsBitmap;
+
+class ImageLoaderPlugin {
+public:
+    virtual ~ImageLoaderPlugin() {}
+
+    virtual Size size() = 0;
+    virtual RefPtr<GraphicsBitmap> bitmap() = 0;
+
+protected:
+    ImageLoaderPlugin() {}
+};
+
+class ImageLoader : public RefCounted<ImageLoader> {
+public:
+    static NonnullRefPtr<ImageLoader> create(const u8* data, size_t size) { return adopt(*new ImageLoader(data, size)); }
+    ~ImageLoader();
+
+    Size size() const { return m_plugin->size(); }
+    int width() const { return size().width(); }
+    int height() const { return size().height(); }
+    RefPtr<GraphicsBitmap> bitmap() const { return m_plugin->bitmap(); }
+
+private:
+    ImageLoader(const u8*, size_t);
+
+    mutable OwnPtr<ImageLoaderPlugin> m_plugin;
+};