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;

src/misc/SDL_url.c

@@ -22,7 +22,7 @@
 
 #include "SDL_sysurl.h"
 
-SDL_bool SDL_OpenURL(const char *url)
+bool SDL_OpenURL(const char *url)
 {
     if (!url) {
         return SDL_InvalidParamError("url");