Add all missing "is characteristic" stdlib functions

SDL has been missing a bunch of these 'isX' functions for some time,
where X is some characteristic of a given character.

This commit adds the rest of them to the SDL stdlib, so now we have:
- SDL_isalpha()
- SDL_isalnum()
- SDL_isblank()
- SDL_iscntrl()
- SDL_isxdigit()
- SDL_ispunct()
- SDL_isprint()
- SDL_isgraph()

src/dynapi/SDL_dynapi_overrides.h

@@ -799,3 +799,11 @@
 #define SDL_SetWindowMouseGrab SDL_SetWindowMouseGrab_REAL
 #define SDL_GetWindowKeyboardGrab SDL_GetWindowKeyboardGrab_REAL
 #define SDL_GetWindowMouseGrab SDL_GetWindowMouseGrab_REAL
+#define SDL_isalpha SDL_isalpha_REAL
+#define SDL_isalnum SDL_isalnum_REAL
+#define SDL_isblank SDL_isblank_REAL
+#define SDL_iscntrl SDL_iscntrl_REAL
+#define SDL_isxdigit SDL_isxdigit_REAL
+#define SDL_ispunct SDL_ispunct_REAL
+#define SDL_isprint SDL_isprint_REAL
+#define SDL_isgraph SDL_isgraph_REAL