add support for custom Java Drawables

This is made possible, by adding a second Canvas implementation which
can be used to render directly to GdkSnapshot objects

For now the only implemented method is drawBitmap(), this is already
enough to make VectorDrawableCompat functional

src/api-impl-jni/graphics/android_graphics_GskCanvas.c

@@ -0,0 +1,16 @@
+#include <gtk/gtk.h>
+#include <graphene.h>
+
+#include "../defines.h"
+#include "../util.h"
+
+#include "../generated_headers/android_graphics_GskCanvas.h"
+
+JNIEXPORT void JNICALL Java_android_graphics_GskCanvas_native_1drawBitmap(JNIEnv *env, jclass this_class, jlong snapshot_ptr, jlong pixbuf_ptr, jint x, jint y, jint width, jint height)
+{
+	GdkSnapshot *snapshot = (GdkSnapshot *)_PTR(snapshot_ptr);
+	GdkPixbuf *pixbuf = (GdkPixbuf *)_PTR(pixbuf_ptr);
+	GdkTexture *texture = gdk_texture_new_for_pixbuf(pixbuf);
+	gtk_snapshot_append_texture(snapshot, texture, &GRAPHENE_RECT_INIT(x, y, width, height));
+	g_object_unref(texture);
+}