Use unbiased pixel rounding for borders

[zcyzcy88]

Same as https://github.com/MoonchildProductions/Pale-Moon/issues/1556

[Lorienna]

Regarding the PM team's solution...

The following commit solved the issue #1556 in the deprecated repo of Pale Moon 27 (ESR-38 hard fork with the classic UI):

• "Round subpixel border widths to nearest integer instead of nearest-below integer."
  https://github.com/MoonchildProductions/Pale-Moon/commit/7d810df6d25093d0985c97525abd4914a787ae8b

As a consequence, caret became thick, and people complained:

• Issue #1575: "Split off caret width from CSS border width and give it the anaemic treatment"
  https://github.com/MoonchildProductions/Pale-Moon/issues/1575

• Commit: "Split border and caret width rounding."
  https://github.com/MoonchildProductions/Pale-Moon/commit/c3b23d899c92037a49c4f8cd669069e5391ab42b

The patches for #1556 and #1575 were later ported to the Unified XUL Platform (UXP; ESR-52 hard fork):

• Commit: "Use natural border width rounding."
  https://github.com/MoonchildProductions/UXP/commit/d1b17ef97efd0e4f869c98dfada8d09f1cb1ad5a

Also, as a side-effect of rounding sub-pixel border widths, tabs in Basilisk (UXP-based browser with Australis UI) became afflicted with vertical lines in HiDPI mode:

• Issue #268: "[BASILISK] Vertical stripes in tabs at high DPI display resolutions"
  https://github.com/MoonchildProductions/UXP/issues/268

• Commit: "Create a 1px overlap between tab center and start/end backgrounds."
  https://github.com/MoonchildProductions/UXP/commit/32c067857c532bf651e5e6946ec1a67772eb2252

As for the possible solution from the old PM repo suggested in #538, I believe that's irrelevant to Australis theme:

• Issue #1584: "Adjust theme to account for natural border width scaling"
  https://github.com/MoonchildProductions/Pale-Moon/issues/1584

• Commit: "Adjust the theme to account for natural border width rounding."
  https://github.com/MoonchildProductions/Pale-Moon/commit/afa34c7a6978ac5c61797d5a875f34601e900def

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Maybe someone can give this a proper testing in Waterfox.