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/initkbd.c

@@ -273,6 +273,7 @@ void set_kbd_iopointers() {
 #define KB_HP9000 (10 + MIN_KEYTYPE)  /* TODO: Can we remove this? */
 #define KB_X (11 + MIN_KEYTYPE)
 #define KB_DOS (12 + MIN_KEYTYPE)
+#define KB_SDL (13 + MIN_KEYTYPE)
 
 /* KB_SUN4 not defined in older OS versions */
 #ifndef KB_SUN4
@@ -432,6 +433,8 @@ void keyboardtype(int fd)
     type = KB_X;
 #elif DOS
     type = KB_DOS;
+#elif SDL
+    type = KB_SDL;
 #endif /* XWINDOW */
   } /* if end */
   else {
@@ -447,6 +450,8 @@ void keyboardtype(int fd)
       type = KB_X;
     else if (strcmp("x", key) == 0)
       type = KB_X;
+    else if (strcmp("sdl", key) == 0)
+      type = KB_SDL;
     else
       type = KB_SUN3; /* default */
   }
@@ -483,7 +488,11 @@ void keyboardtype(int fd)
       InterfacePage->devconfig |= KB_SUN3 - MIN_KEYTYPE; /* 10 */
       break;
 #endif /* XWINDOW */
-
+#ifdef SDL
+  case KB_SDL:
+    InterfacePage->devconfig |= KB_SUN3 - MIN_KEYTYPE; /* 10 */
+    break;
+#endif /* SDL */
 #ifdef DOS
     case KB_DOS:
       SUNLispKeyMap = DOSLispKeyMap_101;