Doubleclick is not correctly set for InputEventMouseButton events on mobile touch screens

[andy-noisyduck]

Godot version:
3.2.3

OS/device including version:
Desktop test was Win 10.
Mobile test Android 11 on Galaxy S10e

Issue description:
The doubleclick property for an InputEventMouseButton event is not correctly set on mobile devices when the source is a Control.

Steps to reproduce:
With default project settings (i.e. "Emulate Touch As Mouse" is true):

1. Override _gui_input for a Control node with mouse flags correctly set.
2. Print the input events, including the doubleclick parameter.

On Desktop the output is for a double click is (4 events generated, press and release for each touch in the double click):

Event doubleclick = False, pressed = True
Event doubleclick = False, pressed = False
Event doubleclick = True, pressed = True
Event doubleclick = False, pressed = False

The same test on mobile gives the following output:

Event doubleclick = False, pressed = True
Event doubleclick = False, pressed = False
Event doubleclick = False, pressed = True
Event doubleclick = False, pressed = False

Notice the second press does not trigger a double click.

Minimal reproduction project:
Testing-DoubleClickControl.zip

[Calinou]

I was able to reproduce this in a project of mine, although I've gotten doubleclick to work on mobile if the touch position was exactly the same between the two presses. This is very difficult to do on such hiDPI displays and fingers 🙂

We probably need to add a "touch distance tolerance" for double-click events to fix this. This distance should be resolution-independent and should correlate to the device's physical size as much as we reasonably can.

[m4gr3d]

cc @thebestnom