fix: race condition on repaint with alt screen

I didn't realise at the time, but the tea.Program and the renderer share
the mutex. This make the code difficult to reason about - it turns out
the program sometimes acquires the lock and then calls the
`setAltScreen` method of the renderer which in turns calls `repaint`.
That causes a deadlock as `repaint` is trying to acquire the lock too.

standard_renderer.go

@@ -230,9 +230,7 @@ func (r *standardRenderer) write(s string) {
 }
 
 func (r *standardRenderer) repaint() {
-	r.mtx.Lock()
 	r.lastRender = ""
-	r.mtx.Unlock()
 }
 
 func (r *standardRenderer) altScreen() bool {
@@ -350,7 +348,9 @@ func (r *standardRenderer) handleMessages(msg Msg) {
 	case repaintMsg:
 		// Force a repaint by clearing the render cache as we slide into a
 		// render.
+		r.mtx.Lock()
 		r.repaint()
+		r.mtx.Unlock()
 
 	case WindowSizeMsg:
 		r.mtx.Lock()
@@ -363,7 +363,9 @@ func (r *standardRenderer) handleMessages(msg Msg) {
 
 		// Force a repaint on the area where the scrollable stuff was in this
 		// update cycle
+		r.mtx.Lock()
 		r.repaint()
+		r.mtx.Unlock()
 
 	case syncScrollAreaMsg:
 		// Re-render scrolling area
@@ -372,7 +374,9 @@ func (r *standardRenderer) handleMessages(msg Msg) {
 		r.insertTop(msg.lines, msg.topBoundary, msg.bottomBoundary)
 
 		// Force non-scrolling stuff to repaint in this update cycle
+		r.mtx.Lock()
 		r.repaint()
+		r.mtx.Unlock()
 
 	case scrollUpMsg:
 		r.insertTop(msg.lines, msg.topBoundary, msg.bottomBoundary)

tea.go

@@ -503,7 +503,9 @@ func (p *Program) StartReturningModel() (Model, error) {
 				continue
 
 			case WindowSizeMsg:
+				p.mtx.Lock()
 				p.renderer.repaint()
+				p.mtx.Unlock()
 
 			case enterAltScreenMsg:
 				p.EnterAltScreen()