thread: Bug fix: Queue funcs instead of panic

Now the thread object is created at (*UserInterface).Run, we don't
have to care whether the (main) thread is started or not when
Call is called. Admit queueing the functions.

Fixes #884

internal/thread/thread.go

@@ -16,13 +16,11 @@ package thread
 
 import (
 	"context"
-	"sync/atomic"
 )
 
 // Thread represents an OS thread.
 type Thread struct {
-	started int32
+	funcs chan func()
-	funcs   chan func()
 }
 
 // New creates a new thread.
@@ -38,9 +36,6 @@ func New() *Thread {
 //
 // Loop must be called on the thread.
 func (t *Thread) Loop(context context.Context) {
-	atomic.StoreInt32(&t.started, 1)
-	defer atomic.StoreInt32(&t.started, 0)
-
 loop:
 	for {
 		select {
@@ -56,10 +51,6 @@ loop:
 //
 // Do not call this from the same thread. This would block forever.
 func (t *Thread) Call(f func() error) error {
-	if atomic.LoadInt32(&t.started) == 0 {
-		panic("thread: the thread loop is not started yet")
-	}
-
 	ch := make(chan struct{})
 	var err error
 	t.funcs <- func() {

internal/uidriver/glfw/ui.go

@@ -172,13 +172,6 @@ func getCachedMonitor(wx, wy int) (*cachedMonitor, bool) {
 	return nil, false
 }
 
-func (u *UserInterface) mainThreadLoop(context context.Context) error {
-	u.setRunning(true)
-	u.t.Loop(context)
-	u.setRunning(false)
-	return nil
-}
-
 func (u *UserInterface) isRunning() bool {
 	u.m.Lock()
 	v := u.running
@@ -575,7 +568,9 @@ func (u *UserInterface) Run(width, height int, scale float64, title string, uico
 		}
 	}()
 
-	u.mainThreadLoop(ctx)
+	u.setRunning(true)
+	u.t.Loop(ctx)
+	u.setRunning(false)
 	return <-ch
 }