Integrated Phil Hassey's patch to fix SDL_IsScreenKeyboardShown on iOS (https://bugzilla.libsdl.org/show_bug.cgi?id=2660)

src/video/uikit/SDL_uikitview.h

@@ -49,7 +49,7 @@
 - (void)hideKeyboard;
 - (void)initializeKeyboard;
 
-@property (nonatomic, readonly, assign, getter=isKeyboardVisible) BOOL keyboardVisible;
+@property (nonatomic, assign, getter=isKeyboardVisible) BOOL keyboardVisible;
 @property (nonatomic, assign) SDL_Rect textInputRect;
 @property (nonatomic, assign) int keyboardHeight;
 

src/video/uikit/SDL_uikitview.m

@@ -390,6 +390,7 @@ void _uikit_keyboard_set_height(int height) {
         return;
     }
 
+    view.keyboardVisible = height > 0;
     view.keyboardHeight = height;
     _uikit_keyboard_update();
 }