gnu: ffmpeg@4: Build against SDL2 2.0 so 'ffplay' gets built.

Fixes <https://issues.guix.gnu.org/58090>.
Reported by Zhu Zihao <all_but_last@163.com>.

* gnu/packages/sdl.scm (sdl2-2.0): New variable.
* gnu/packages/video.scm (ffmpeg-4)[inputs]: New field.

gnu/packages/sdl.scm

@@ -155,6 +155,20 @@ joystick, and graphics hardware.")
                wayland-protocols)))
     (license license:bsd-3)))
 
+(define-public sdl2-2.0
+  (package
+    (inherit sdl2)
+    (name "sdl2")
+    (version "2.0.14")
+    (source (origin
+              (method url-fetch)
+              (uri
+               (string-append "https://libsdl.org/release/SDL2-"
+                              version ".tar.gz"))
+              (sha256
+               (base32
+                "1g1jahknv5r4yhh1xq5sf0md20ybdw1zh1i15lry26sq39bmn8fq"))))))
+
 (define-public libmikmod
   (package
     (name "libmikmod")