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/ldeboot.c

@@ -35,7 +35,9 @@
 #include <X11/Xutil.h>
 #define LDEX "ldex"
 #endif /* XWINDOW */
-
+#ifdef SDL
+#define LDESDL "ldesdl"
+#endif
 #define LDEMONO "ldesingle"
 #define LDECOLOR "ldemulti"
 #define LDETRUECOLOR "ldetruecolor"
@@ -187,7 +189,7 @@ int main(int argc, char *argv[])
     fork_Unix();
 
   /* start ldemono or ldecolor */
-
+  strcpy(filetorun,LDESDL);
   if (filetorun[0] == '\0') {
     fprintf(stderr, "Unable to determine what display program to run.\n");
     exit(1);