For now, only support 1x and 2x scale.
I tried doing something "smarter" first where the UI would try
to keep the physical resolution constant when toggling between
1x and 2x, but many of the smaller 1x resolutions map to 2x
logical resolutions that Compositor rejects (e.g. 1024x768 becomes
512x384, which is less than the minimum 640x480 that Compositor
wants) and it felt complicated and overly magical.
So this instead just gives you a 1x/2x toggle and a dropdown
with logical (!) resolutions. That is, 800x600 @ 2x gives you
a physical resolution of 1600x1200.
If we don't like this after trying it for a while, we can change
the UI then.
Now that WindowServer broadcasts the system theme using an anonymous
file, we need clients to pledge "recvfd" so they can receive it.
Some programs keep the "shared_buffer" pledge since it's still used for
a handful of things.
Now, `chres 640 480 2` can set the UI to HighDPI 640x480 at runtime. A
real GUI for changing the display factor will come later.
(`chres 640 480 2` followed by `chres 1280 960` is very fast since
we don't have to re-allocate the framebuffer since both modes use
the exact same number of physical pixels.)
