fix: Set button size in onMount if not set (#3413)

If the buttons are extended it doesn't automatically propagate the size
from the `button` child component like it does if you pass the button
into the constructor. This PR sets that size in `onMount` if the size is
zero.

packages/flame/lib/src/components/input/button_component.dart

@@ -45,6 +45,9 @@ class ButtonComponent extends PositionComponent with TapCallbacks {
       button != null,
       'The button has to either be passed in as an argument or set in onLoad',
     );
+    if (size.isZero()) {
+      size = button!.size;
+    }
     if (!contains(button!)) {
       add(button!);
     }

packages/flame/lib/src/components/input/sprite_button_component.dart

@@ -65,6 +65,9 @@ class SpriteButtonComponent extends SpriteGroupComponent<ButtonState>
       _button != null,
       'The button sprite has to be set either in onLoad or in the constructor',
     );
+    if (size.isZero()) {
+      size = _button!.originalSize;
+    }
     sprites = {
       ButtonState.up: _button!,
       ButtonState.down: buttonDown,