Draw focus ring as border on SSD windows

With SSD we're reasonably sure that the window is a rectangle with no
rounded corners, so a rectangular border should work.

Cargo.lock

@@ -121,6 +121,12 @@ dependencies = [
  "num-traits",
 ]
 
+[[package]]
+name = "arrayvec"
+version = "0.7.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "96d30a06541fbafbc7f82ed10c06164cfbd2c401138f6addd8404629c4b16711"
+
 [[package]]
 name = "async-broadcast"
 version = "0.5.1"
@@ -1462,6 +1468,7 @@ name = "niri"
 version = "0.1.0"
 dependencies = [
  "anyhow",
+ "arrayvec",
  "async-channel",
  "async-io",
  "bitflags 2.4.0",