Removed SDL_bool in favor of plain bool

We require stdbool.h in the build environment, so we might as well use the plain bool type.

If your environment doesn't have stdbool.h, this simple replacement will suffice:
typedef signed char bool;

test/testpen.c

@@ -23,8 +23,8 @@ typedef struct Pen
     float x;
     float y;
     Uint32 buttons;
-    SDL_bool eraser;
-    SDL_bool touching;
+    bool eraser;
+    bool touching;
     struct Pen *next;
 } Pen;
 
@@ -150,7 +150,7 @@ SDL_AppResult SDL_AppEvent(void *appstate, SDL_Event *event)
             /*SDL_Log("Pen %" SDL_PRIu32 " down!", event->ptouch.which);*/
             pen = FindPen(event->ptouch.which);
             if (pen) {
-                pen->touching = SDL_TRUE;
+                pen->touching = true;
                 pen->eraser = (event->ptouch.eraser != 0);
             }
             return SDL_APP_CONTINUE;
@@ -159,7 +159,7 @@ SDL_AppResult SDL_AppEvent(void *appstate, SDL_Event *event)
             /*SDL_Log("Pen %" SDL_PRIu32 " up!", event->ptouch.which);*/
             pen = FindPen(event->ptouch.which);
             if (pen) {
-                pen->touching = SDL_FALSE;
+                pen->touching = false;
                 pen->axes[SDL_PEN_AXIS_PRESSURE] = 0.0f;
             }
             return SDL_APP_CONTINUE;