render: add cross-platform desktop companion display

tests/engine/desktop_second_screen.lua

@@ -0,0 +1,57 @@
+package.path = "./?.lua;./?/init.lua;" .. package.path
+
+local clock = 1
+love = {
+  timer = { getTime = function() return clock end },
+  data = {
+    hash = function() return "digest" end,
+    encode = function() return "0123456789abcdef0123456789abcdef" end,
+    compress = function(_, _, value) return value end,
+  },
+}
+
+local sent, spawned, queue = {}, nil, {}
+local peer = {
+  send = function(_, data, channel, flag)
+    sent[#sent + 1] = { data = data, channel = channel, flag = flag }
+  end,
+  disconnect_now = function() end,
+}
+local host = {
+  service = function()
+    if #queue == 0 then return nil end
+    return table.remove(queue, 1)
+  end,
+  destroy = function() end,
+}
+
+package.loaded["src.core.Platform"] = { canSpawnProcess = function() return true end }
+package.loaded["src.core.HostShell"] = {
+  spawnSelfDetached = function(args) spawned = args return true end,
+}
+package.preload.enet = function()
+  return { host_create = function() return host end }
+end
+
+local Screen = require("src.render.SecondScreen")
+assert(Screen.usable(), "the shared facade selects the desktop backend")
+Screen.setEnabled(true)
+assert(spawned and spawned[1]:match("^%-%-display%-companion=%d+,[%w]+$"),
+  "enabling launches one companion of this app")
+local token = spawned[1]:match(",([%w]+)$")
+queue[#queue + 1] = { type = "receive", peer = peer, data = "H" .. token }
+assert(Screen.detected(), "a token-authenticated companion becomes detected")
+
+local pixels = { getString = function() return "rgba" end }
+assert(Screen.push(pixels, 1, 1, 0x102030, "auto"),
+  "a connected companion accepts a frame")
+assert(sent[#sent].data:find("^F" .. token .. "\n1,1,1056816,auto\nrgba"),
+  "frame metadata and pixels stay in one loopback packet")
+
+queue[#queue + 1] = {
+  type = "receive", peer = peer, data = "I" .. token .. "\ndown,3,4",
+}
+assert(Screen.pollTouch() == "down,3,4", "companion input returns to the mod")
+Screen.setEnabled(false)
+assert(sent[#sent].data == "Q" .. token, "disabling closes the companion")
+print("desktop second screen: ok")