The OpenGL 4.6 specification §14.5.1 "Basic Line Rasterization"
figure 14.2 says:
"""A diamond shaped region of height 1 is placed around each fragment
center; those regions that the line segment **exits** cause
rasterization to produce corresponding fragments."""
As the line does not necessarily exit the last diamond,
it is necessary to explicitly paint a pixel at line ends.
Fixes: https://gitlab.freedesktop.org/xorg/xserver/-/issues/1434
Part-of: <https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/1755>