IME Composition Truncation + SDL_IsTextInputShown + SDL_ClearComposition (#5398)

* Fixes for IME Composition Truncation + Addition of SDL_ClearComposition, SDL_IsTextInputShown

* Fixed: Documentation and code style issues raised during code review.

include/SDL_keyboard.h

@@ -268,6 +268,24 @@ extern DECLSPEC SDL_bool SDLCALL SDL_IsTextInputActive(void);
  */
 extern DECLSPEC void SDLCALL SDL_StopTextInput(void);
 
+/**
+ * Dismiss the composition window/IME without disabling the subsystem.
+ *
+ * \since This function is available since SDL 2.0.22
+ *
+ * \sa SDL_StartTextInput
+ * \sa SDL_StopTextInput
+ */
+extern DECLSPEC void SDLCALL SDL_ClearComposition(void);
+
+/**
+ * Returns if an IME Composite or Candidate window is currently shown.
+ *
+ * \since This function is available since SDL 2.0.22
+ *
+ */
+extern DECLSPEC SDL_bool SDLCALL SDL_IsTextInputShown(void);
+
 /**
  * Set the rectangle used to type Unicode text inputs.
  *