app: [Windows] avoid default IME composition window

Mark WM_IME_STARTCOMPOSITION as handled after positioning the IME
composition and candidate windows.

Passing the message on to DefWindowProc can let Windows create its default
composition window, which shows the first composing character below Gio's
caret.

Fixes: https://todo.sr.ht/~eliasnaur/gio/687
Signed-off-by: qiannian <qianniancn@gmail.com>

app/os_windows.go

@@ -412,6 +412,7 @@ func windowProc(hwnd syscall.Handle, msg uint32, wParam, lParam uintptr) uintptr
 		icaret := image.Pt(int(caret.X+.5), int(caret.Y+.5))
 		windows.ImmSetCompositionWindow(imc, icaret.X, icaret.Y)
 		windows.ImmSetCandidateWindow(imc, icaret.X, icaret.Y)
+		return windows.TRUE
 	case windows.WM_IME_COMPOSITION:
 		imc := windows.ImmGetContext(w.hwnd)
 		if imc == 0 {