render: add cross-platform desktop companion display

tests/engine/desktop_companion.lua

@@ -0,0 +1,56 @@
+package.path = "./?.lua;./?/init.lua;" .. package.path
+
+local clock, quit = 1, false
+local sent, queue, draws = {}, {}, 0
+local peer = {
+  send = function(_, data) sent[#sent + 1] = data end,
+  disconnect_now = function() end,
+}
+local host = {
+  connect = function() return peer end,
+  service = function()
+    if #queue == 0 then return nil end
+    return table.remove(queue, 1)
+  end,
+}
+local rendered = {
+  setFilter = function() end, replacePixels = function() end,
+  release = function() end,
+}
+love = {
+  timer = { getTime = function() return clock end },
+  data = { decompress = function(_, _, value) return value end },
+  image = { newImageData = function(w, h, format, raw)
+    assert(w == 1 and h == 1 and format == "rgba8" and raw == "rgba")
+    return {}
+  end },
+  graphics = {
+    newImage = function() return rendered end,
+    getDimensions = function() return 100, 100 end,
+    clear = function() end, setColor = function() end,
+    draw = function() draws = draws + 1 end,
+  },
+  event = { quit = function() quit = true end },
+}
+package.preload.enet = function()
+  return { host_create = function() return host end }
+end
+
+require("src.render.DesktopCompanion").install({ port = 50000, token = "token" })
+queue[#queue + 1] = { type = "connect" }
+love.update()
+assert(sent[#sent] == "Htoken", "companion authenticates after connecting")
+queue[#queue + 1] = {
+  type = "receive", data = "Ftoken\n1,1,0,auto\nrgba",
+}
+love.update()
+love.draw()
+assert(draws == 1, "companion draws a received frame")
+love.mousepressed(50, 50, 1)
+love.mousereleased(50, 50, 1)
+assert(sent[#sent - 1] == "Itoken\ndown,0,0"
+  and sent[#sent] == "Itoken\nup,0,0", "mouse input maps back to source pixels")
+queue[#queue + 1] = { type = "receive", data = "Qtoken" }
+love.update()
+assert(quit, "parent can close the companion")
+print("desktop companion: ok")