Allow push_input() to work when gui_disable_input is enabled on Viewports

[YeldhamDev]

In 3.*, if a Viewport had gui_disable_input enabled, it could still receive inputs via the input() method. This was handy if you wished to cherry-pick which inputs went into a sub-viewport (which is the case in my game).

In 4.*, however, this is not possible anymore. So this PR brings back the old behavior.

[akien-mga]

CC @Sauermann

[Sauermann]

I believe, that the suggested API is not ideal. My reasons for this are:

• gui_disable_input was previously a Viewport-internal flag. With this change, callers of push_input now need to additionally check and respect the setting of gui_disable_input. This seems to be an unnecessary complication and seems unexpected.
• The use-case showcases a missing functionality of SubViewportContainer and it is solvable without any changes to Viewport like in Enable InputEvent-filtering in SubViewportContainer #78762, so a local solution seems to be more fitting.

[YeldhamDev]

Sounds even better! Closing in favor of that alternative.