Use dep to vendor things

vendor/github.com/faiface/mainthread/LICENSE

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+MIT License
+
+Copyright (c) 2017 Michal Štrba
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.

vendor/github.com/faiface/mainthread/README.md

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+# mainthread [![GoDoc](https://godoc.org/github.com/faiface/mainthread?status.svg)](http://godoc.org/github.com/faiface/mainthread) [![Report card](https://goreportcard.com/badge/github.com/faiface/mainthread)](https://goreportcard.com/report/github.com/faiface/mainthread)
+
+Package mainthread allows you to run code on the main operating system thread.
+
+`go get github.com/faiface/mainthread`
+
+Operating systems often require, that code which deals with windows and graphics has to run on the
+main thread. This is however somehow challenging in Go due to Go's concurrent nature.
+
+This package makes it easily possible.
+
+All you need to do is put your main code into a separate function and call `mainthread.Run` from
+your real main, like this:
+
+```go
+package main
+
+import (
+	"fmt"
+
+	"github.com/faiface/mainthread"
+)
+
+func run() {
+	// now we can run stuff on the main thread like this
+	mainthread.Call(func() {
+		fmt.Println("printing from the main thread")
+	})
+	fmt.Println("printing from another thread")
+}
+
+func main() {
+	mainthread.Run(run) // enables mainthread package and runs run in a separate goroutine
+}
+```
+
+## More functions
+
+If you don't wish to wait until a function finishes running on the main thread, use
+`mainthread.CallNonBlock`:
+
+```go
+mainthread.CallNonBlock(func() {
+	fmt.Println("i'm in the main thread")
+})
+fmt.Println("but imma be likely printed first, cuz i don't wait")
+```
+
+If you want to get some value returned from the main thread, you can use `mainthread.CallErr` or
+`mainthread.CallVal`:
+
+```go
+err := mainthread.CallErr(func() error {
+	return nil // i don't do nothing wrong
+})
+val := mainthread.CallVal(func() interface{} {
+	return 42 // the meaning of life, universe and everything
+})
+```
+
+If `mainthread.CallErr` or `mainthread.CallVal` aren't sufficient for you, you can just assign
+variables from within the main thread:
+
+```go
+var x, y int
+mainthread.Call(func() {
+	x, y = 1, 2
+})
+```
+
+However, be careful with `mainthread.CallNonBlock` when dealing with local variables.

vendor/github.com/faiface/mainthread/mainthread.go

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+package mainthread
+
+import (
+	"errors"
+	"runtime"
+)
+
+// CallQueueCap is the capacity of the call queue. This means how many calls to CallNonBlock will not
+// block until some call finishes.
+//
+// The default value is 16 and should be good for 99% usecases.
+var CallQueueCap = 16
+
+var (
+	callQueue chan func()
+)
+
+func init() {
+	runtime.LockOSThread()
+}
+
+func checkRun() {
+	if callQueue == nil {
+		panic(errors.New("mainthread: did not call Run"))
+	}
+}
+
+// Run enables mainthread package functionality. To use mainthread package, put your main function
+// code into the run function (the argument to Run) and simply call Run from the real main function.
+//
+// Run returns when run (argument) function finishes.
+func Run(run func()) {
+	callQueue = make(chan func(), CallQueueCap)
+
+	done := make(chan struct{})
+	go func() {
+		run()
+		done <- struct{}{}
+	}()
+
+	for {
+		select {
+		case f := <-callQueue:
+			f()
+		case <-done:
+			return
+		}
+	}
+}
+
+// CallNonBlock queues function f on the main thread and returns immediately. Does not wait until f
+// finishes.
+func CallNonBlock(f func()) {
+	checkRun()
+	callQueue <- f
+}
+
+// Call queues function f on the main thread and blocks until the function f finishes.
+func Call(f func()) {
+	checkRun()
+	done := make(chan struct{})
+	callQueue <- func() {
+		f()
+		done <- struct{}{}
+	}
+	<-done
+}
+
+// CallErr queues function f on the main thread and returns an error returned by f.
+func CallErr(f func() error) error {
+	checkRun()
+	errChan := make(chan error)
+	callQueue <- func() {
+		errChan <- f()
+	}
+	return <-errChan
+}
+
+// CallVal queues function f on the main thread and returns a value returned by f.
+func CallVal(f func() interface{}) interface{} {
+	checkRun()
+	respChan := make(chan interface{})
+	callQueue <- func() {
+		respChan <- f()
+	}
+	return <-respChan
+}