Fix font metrics and switch to windows subsystem

Currently fonts are using incorrect metrics and the app uses the console
subsystem. This patch adds aliases so that font metrics are found and
instructs the linker to use the windows subsystem.

python/servo/build_commands.py

@@ -236,6 +236,9 @@ class MachCommands(CommandBase):
 
         cargo_binary = "cargo" + BIN_SUFFIX
 
+        if sys.platform == "win32" or sys.platform == "msys":
+            env["RUSTFLAGS"] = "-C link-args=-Wl,--subsystem,windows"
+
         status = call(
             [cargo_binary, "build"] + opts,
             env=env, cwd=self.servo_crate(), verbose=verbose)