render: add cross-platform desktop companion display

tests/engine/hostshell_spawn_self.lua

@@ -0,0 +1,32 @@
+package.path = "./?.lua;./?/init.lua;" .. package.path
+
+local osName, fused, command = "Windows", true, nil
+love = {
+  system = { getOS = function() return osName end },
+  filesystem = {
+    getExecutablePath = function() return "C:\\Game\\gen1recomp.exe" end,
+    getSource = function() return "C:\\Source\\gen1recomp" end,
+    isFused = function() return fused end,
+  },
+}
+package.loaded["src.core.Platform"] = { canSpawnProcess = function() return true end }
+local execute = os.execute
+os.execute = function(value) command = value return 0 end
+
+local HostShell = require("src.core.HostShell")
+assert(HostShell.spawnSelfDetached({ "--display-companion=50000,token" }))
+assert(command:find('start "" /b ', 1, true)
+  and command:find('"C:\\Game\\gen1recomp.exe"', 1, true),
+  "Windows launches the fused app detached")
+
+osName, fused = "Linux", false
+assert(HostShell.spawnSelfDetached({ "--display-companion=50000,token" }))
+assert(command:find("'C:\\Source\\gen1recomp'", 1, true)
+  and command:sub(-1) == "&", "Linux source runs include the game folder")
+
+osName, fused = "OS X", true
+assert(HostShell.spawnSelfDetached({ "--display-companion=50000,token" }))
+assert(not command:find("start", 1, true) and command:sub(-1) == "&",
+  "macOS uses the same detached POSIX path")
+os.execute = execute
+print("spawn self detached: ok")