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