Include mouse back and forward buttons on Linux mouse events.

[2bndy5]

Like the title says, this includes the mouse's backward and forward buttons reported mouse events.

resolves flutter/flutter#152128

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[robert-ancell]

This repository is no longer in use - this change should be proposed to https://github.com/flutter/flutter which now contains the engine.

[robert-ancell]

I did review the change, however I'm not sure if buttons 4 and 5 map to back and forward on all mice. Looking at the GTK source it seems buttons 4-7 map to scroll up/down/left/right on X11, and Wayland it states:

/* For compatibility reasons, all additional buttons go after the old 4-7 scroll ones */
gdk_button = button - BUTTON_BASE + 4;

Which I think is incorrect, as this will still set the fourth button will still be set to ID 4.

In Flutters embedder.h it states:

/// If a mouse has more than five buttons, send higher bit shifted values
/// corresponding to the button number: 1 << 5 for the 6th, etc.

which I think we should also be doing for any remaining buttons.

[2bndy5]

I finally got around to testing this...

Apparently, the mouse back and forward buttons register as 8 & 9 respectively (in GdkEventButton.button). So, the following patch actually works as expected (using et run on a simple flutter project):

    case 8:
      *button = kFlutterPointerButtonMouseBack;
      return TRUE;
    case 9:
      *button = kFlutterPointerButtonMouseForward;
      return TRUE;