cl-SDL2 appears to block SLIME/swank updates in event loop

[fouric]

The function swank::handle-requests appears to become non-functional in an SDL2 event loop. Example code:

(defun test ()
  (sdl2:with-init (:everything)
    (sdl2:with-window (window)
      (sdl2:with-renderer (renderer window :flags '(:presentvsync))
        (sdl2:with-event-loop (:method :poll)
          (:idle ()
                 (let ((connection (or swank::*emacs-connection* (swank::default-connection))))
                   (when connection
                     (swank::handle-requests connection t)))
                 (sdl2:set-render-draw-color renderer 0 0 0 255)
                 (sdl2:render-clear renderer)
                 (sdl2:render-present renderer))
          (:quit () t))))))

In the above code, the SLIME REPL does not update while the SDL2 event loop is running. When I run another loop in the same image that does not invoke the SDL2 event loop, the SLIME REPL updates successfully:

```lisp
(defun test-2 ()
  (dotimes (i 10)
    (sleep 1)
    (let ((connection (or swank::*emacs-connection* (swank::default-connection))))
      (when connection
        (swank::handle-requests connection t))))
  (princ "done"))

Environment:
`sbcl --version`: SBCL 2.0.1.debian
`uname -a`: Linux monolith 5.4.0-72-generic #80-Ubuntu SMP Mon Apr 12 17:35:00 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux
emacs `version`: GNU Emacs 26.3 (build 2, x86_64-pc-linux-gnu, GTK+ Version 3.24.14) of 2020-03-26, modified by Debian
cl-sdl2 version: cl-sdl2-20201220-git