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;

docs/README-dynapi.md

@@ -35,7 +35,7 @@ SDL now has, internally, a table of function pointers. So, this is what SDL_Init
 now looks like:
 
 ```c
-SDL_bool SDL_Init(SDL_InitFlags flags)
+bool SDL_Init(SDL_InitFlags flags)
 {
     return jump_table.SDL_Init(flags);
 }
@@ -49,7 +49,7 @@ SDL_Init() that you've been calling all this time. But at startup, it looks more
 like this:
 
 ```c
-SDL_bool SDL_Init_DEFAULT(SDL_InitFlags flags)
+bool SDL_Init_DEFAULT(SDL_InitFlags flags)
 {
     SDL_InitDynamicAPI();
     return jump_table.SDL_Init(flags);