Add prototypical SDL support.

Build with cmake. This will create a new backend (ldesdl).
- Resolution can only be set by editing the variables in sdl.c.
- Key repeat does not work.
- Still problems with keysyms that implicitly contain modifiers.
- The entire screen is bitblted onto the SDL display every frame.

Support keyboard, and work on mouse.

Kind of working...

Fix display resolution problems.

src/main.c

@@ -515,7 +515,9 @@ int main(int argc, char *argv[])
 #if defined(DOS) || defined(XWINDOW)
   make_dsp_instance(currentdsp, 0, 0, 0, 1); /* All defaults the first time */
 #endif                                       /* DOS || XWINDOW */
-
+#if defined(SDL)
+  init_SDL();
+#endif /* SDL */
   /* Load sysout to VM space and returns real sysout_size(not 0) */
   sysout_size = sysout_loader(sysout_name, sysout_size);