Godot version:
Godot Engine v3.2.4.rc4.official - https://godotengine.org
OpenGL ES 3.0 Renderer: NVIDIA GeForce GT 650M OpenGL Engine
OpenGL ES Batching: ON
Mac OS X Catalina 10.15.7

Issue description:
VSlider reports old value of slider in _gui_input(event: InputEvent) signal

Steps to reproduce:

• Open New Scene
• Add a VSlider Control
• Add an empty script
• Click on VSlider control and select the Inspector Node Tab
• Connect the gui_input) and _value_changed(() signals to the methods below.
• modify script signal methods as below.
• Run scene

extends VSlider

#This signal is called 1st
func _on_VSlider_gui_input(event: InputEvent) -> void:
	if (event.is_pressed() and event.button_index == BUTTON_LEFT):	
		print("\nClicked")
		print("_gui_input: ",value) # Reports the old slider value

#This signal is called 2nd
func _on_VSlider_value_changed(value: float) -> void:
	print("_value_changed: ",value) # Correctly reports the current slider value

When VSlider is clicked _gui_input() method reports the original old value, while _value_changed() reports the correct new value.

Example Output:

Clicked
_gui_input: 0
_value_changed: 6

Clicked
_gui_input: 6
_value_changed: 1

Clicked
_gui_input: 1
_value_changed: 9

Makes the _gui_input() not as useful.

Thanks!