widgets/terminal: fix damage race condition

Terminal.damage is accessed when drawing and when invalidating the widget. For
this reason we need to protect it with a mutex.

This seems to fix various damage issues I've been experiencing (where some
regions of the terminal weren't correctly repainted).

Race detector trace:

    Read at 0x00c0000c6670 by main goroutine:
      git.sr.ht/~sircmpwn/aerc/widgets.(*Terminal).Draw()
          /home/simon/src/aerc/widgets/terminal.go:292 +0x191
      git.sr.ht/~sircmpwn/aerc/lib/ui.(*Grid).Draw()
          /home/simon/src/aerc/lib/ui/grid.go:117 +0x575
      git.sr.ht/~sircmpwn/aerc/lib/ui.(*Grid).Draw()
          /home/simon/src/aerc/lib/ui/grid.go:117 +0x575
      git.sr.ht/~sircmpwn/aerc/widgets.(*MessageViewer).Draw()
          /home/simon/src/aerc/widgets/msgviewer.go:231 +0x253
      git.sr.ht/~sircmpwn/aerc/lib/ui.(*TabContent).Draw()
          /home/simon/src/aerc/lib/ui/tab.go:124 +0x12e
      git.sr.ht/~sircmpwn/aerc/lib/ui.(*Grid).Draw()
          /home/simon/src/aerc/lib/ui/grid.go:117 +0x575
      git.sr.ht/~sircmpwn/aerc/widgets.(*Aerc).Draw()
          /home/simon/src/aerc/widgets/aerc.go:95 +0x5a
      git.sr.ht/~sircmpwn/aerc/lib/ui.(*UI).Tick()
          /home/simon/src/aerc/lib/ui/ui.go:93 +0x1dd
      main.main()
          /home/simon/src/aerc/aerc.go:105 +0x539

    Previous write at 0x00c0000c6670 by goroutine 37:
      git.sr.ht/~sircmpwn/aerc/widgets.(*Terminal).onDamage-fm()
          /home/simon/src/aerc/widgets/terminal.go:429 +0x131
      git.sr.ht/~sircmpwn/go-libvterm._go_handle_damage()
          /home/simon/go/pkg/mod/git.sr.ht/~sircmpwn/go-libvterm@v0.0.0-20190421201021-3184f6f13687/vterm.go:481 +0xf9
      git.sr.ht/~sircmpwn/go-libvterm._cgoexpwrap_5e22200b58b7__go_handle_damage()
          _cgo_gotypes.go:731 +0x58
      runtime.call32()
          /usr/lib/go/src/runtime/asm_amd64.s:519 +0x3a
      git.sr.ht/~sircmpwn/go-libvterm.(*VTerm).Write.func1()
          /home/simon/go/pkg/mod/git.sr.ht/~sircmpwn/go-libvterm@v0.0.0-20190421201021-3184f6f13687/vterm.go:329 +0x9d
      git.sr.ht/~sircmpwn/go-libvterm.(*VTerm).Write()
          /home/simon/go/pkg/mod/git.sr.ht/~sircmpwn/go-libvterm@v0.0.0-20190421201021-3184f6f13687/vterm.go:329 +0x7f
      git.sr.ht/~sircmpwn/aerc/widgets.NewTerminal.func1()
          /home/simon/src/aerc/widgets/terminal.go:131 +0x18c

    Goroutine 37 (running) created at:
      git.sr.ht/~sircmpwn/aerc/widgets.NewTerminal()
          /home/simon/src/aerc/widgets/terminal.go:121 +0x23f
      git.sr.ht/~sircmpwn/aerc/widgets.NewMessageViewer()
          /home/simon/src/aerc/widgets/msgviewer.go:147 +0xfbe
      git.sr.ht/~sircmpwn/aerc/commands/account.ViewMessage()
          /home/simon/src/aerc/commands/account/view-message.go:26 +0x4a4
      git.sr.ht/~sircmpwn/aerc/commands.(*Commands).ExecuteCommand()
          /home/simon/src/aerc/commands/commands.go:47 +0x1f0
      main.main.func1()
          /home/simon/src/aerc/aerc.go:76 +0x205
      git.sr.ht/~sircmpwn/aerc/widgets.(*Aerc).BeginExCommand.func1()
          /home/simon/src/aerc/widgets/aerc.go:262 +0x89
      git.sr.ht/~sircmpwn/aerc/widgets.(*ExLine).Event()
          /home/simon/src/aerc/widgets/exline.go:47 +0x222
      git.sr.ht/~sircmpwn/aerc/widgets.(*Aerc).Event()
          /home/simon/src/aerc/widgets/aerc.go:133 +0x83c
      git.sr.ht/~sircmpwn/aerc/widgets.(*Aerc).simulate()
          /home/simon/src/aerc/widgets/aerc.go:126 +0x12a
      git.sr.ht/~sircmpwn/aerc/widgets.(*Aerc).Event()
          /home/simon/src/aerc/widgets/aerc.go:148 +0x766
      git.sr.ht/~sircmpwn/aerc/lib/ui.(*UI).Tick()
          /home/simon/src/aerc/lib/ui/ui.go:86 +0x11b
      main.main()
          /home/simon/src/aerc/aerc.go:105 +0x539

0 files changed, 0 insertions, 0 deletions