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/xc.c

@@ -1118,8 +1118,12 @@ check_interrupt:
      * If the system is configured with SIGIO handling we have a hint
      * that allows us to cheaply skip if there's nothing to do
      */
+#ifdef XWINDOW
     process_Xevents(currentdsp);
-
+#endif
+#ifdef SDL
+    process_SDLevents();
+#endif
     if (IO_Signalled) {
       IO_Signalled = FALSE;
       process_io_events();