This patch replaces the use of shbufs for GUI::Window backing stores
with the new anonymous files mechanism.
Backing bitmaps are now built on memory allocated with anon_create().
They are passed across the IPC socket as IPC::File. This means that
WindowServer now pledges "recvfd" and graphical clients need to pledge
"sendfd" to work.
To support the cached bitmap swapping optimization on the WindowServer
side, each backing store is assigned an incrementing serial number on
the client side. (This allows us to re-use an already mapped file.)
Instead of storing the back/front buffers of a GUI::Window as just
Gfx::Bitmap, wrap them in a WindowBackingStore class.
This will allow us to add more information alongside the bitmaps while
keeping the back/front swapping logic simple.
This broke when switching IPC messages to support move-only types.
This pattern is not ideal, but the real fix for this will be using fd
passing instead of shbufs.
Fixes#4955.
This API was a mostly gratuitous deviation from POSIX that gave up some
portability in exchange for avoiding the occasional strlen().
I don't think that was actually achieving anything valuable, so let's
just chill out and have the same open() API as everyone else. :^)
