While an update was forced whenever polybar was made visible, the
`m_lastinput` variable was still set to the same value as when the bar
became hidden because updates to it were prevented.

Fixes #1875

Some WMs like i3 discard position information when unmapping the bar and
because of that the bar would be at the wrong position after being
remapped.

Fixes #1484
Ref: i3/i3#3834

The amdgpu driver seems to set 'actual_brightness' wrong.

Fixes #1870
Ref: Alexays/Waybar#335

Ref: polybar/xpp#17
Fixes #1892

Changelog

Dependencies:

We dropped `python2` as a dependency, just in time for its EOL. Now the only python dependency is `python3` (#1908).

Fixes:

* ipc:
    * Update bar when making it visible (#1889), see #1875
    * Set bar position when making it visible (#1901), see #1484, i3/i3#3834
* `internal/backlight`: Use correct brightness file for `amdgpu_bl0` (#1900), see #1870, Alexays/Waybar#335

Fixes #1881

The issue was that it used the position of the center module to
calculate the leftmost possible position of the block. However, if the
center module is empty that position is disastrously wrong.

Fixes #591
Fixes #1903

Before it did not take into account borders or a tray on the left.
It also sometimes rendered the gradient way to large

The old code didn't really work when the right block was pushing against
the center block. Also `fixed-center` wasn't properly defined. I have
now fixed it to the following:

* `fixed-center = true`: The center block stays at the center of the bar
whenever possible. It can be pushed to the left if the right block takes
too much space and to the right if the left block takes too much space
* `fixed-center = false`: The center block will be in the middle between
the left and right block whenever possible. If there is not enough space
between those two, the center block will be directly to the right of the
left block and pushes out the right block

Before, if the command produced no output, the `m_output` field would
not have been overwritten and the old output was displayed.

But since this is an explicit trigger of the hook, the user would expect
the output to be updated to whatever the script produces (even if that
is nothing).

Ref: https://www.reddit.com/r/Polybar/comments/e9a8ww

This is dependent on a PR to xpp that does the same. Newer compilers
(GCC10 in particular) are stricter about which headers provide the
std exception types.

Fixes

* `internal/i3` (and maybe others): Workspace icon map misbehaves for empty icon (#1893), see #1881
* `custom/ipc`: Clear content if hook produces no output. (#1951)
* renderer:
    * The modules on the right no longer overlap the tray or are shifted out of the bar. (#1936), see #591, #1903, #1838, #1236
    * The gradient when modules are shifted out of the bar (because there is not enough space) is now drawn correctly in all circumstances. (#1936)
* build: Fixed a compilation issue in gcc10 (#1967, polybar/xpp#20)

Fixes: GCC 10 Compilation (#2098), see #2097