ImageDecoder: Add a new service for out-of-process image decoding :^)

The new ImageDecoder service (available for members of "image" via
/tmp/portal/image) allows you to decode images in a separate process.

This will allow programs to confidently load untrusted images, since
the bulk of the security concerns are sandboxed to a separate process.

The only API right now is a synchronous IPC DecodeImage() call that
takes a shbuf with encoded image data and returns a shared buffer and
metadata for the decoded image.

It also comes with a very simple library for interfacing with the
ImageDecoder service: LibImageDecoderClient. The name is a bit of a
mouthful but I guess we can rename it later if we think of something
nicer to call it.

There's obviously a bit of overhead to spawning a separate process
for every image decode, so this is mostly only appropriate for
untrusted images (e.g stuff downloaded from the web) and not necessary
for trusted local images (e.g stuff in /res)

Services/ImageDecoder/ImageDecoderClient.ipc

@@ -0,0 +1,4 @@
+endpoint ImageDecoderClient = 7002
+{
+    Dummy() =|
+}