Revert to Stretch's xorg-xserver

For #12219 (closed) we’ve tried upgrading to xorg-xserver 2:1.19.3-1 but that did not fix the bug. Since then we’ve stuck to that version, which has a greater version that the one in Stretch, but 1. does not get any security updates; 2. does not track new versions from testing/sid either. I think we should drop our custom package in Tails 3.6 and track the version in Stretch instead. I’ll send a call for testing to help ensure this downgrade does not bring too many regressions.

The 1.19.2..1.19.3 changes can be seen there: https://cgit.freedesktop.org/xorg/xserver/log/?h=xorg-server-1.19.3

Feature Branch: bugfix/15232-revert-to-stretch-xorg-xserver

Related issues

Blocks #13245 (closed)
Blocked by #15270 (closed)

Original created by @intrigeri on 15232 (Redmine)

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