Add antialiasing to traffic graph widget

Summary: Backport of Core PR16153

Test Plan:
Visual improvement on network traffic plot:
src/qt/bitcoin-qt -> Help -> Debug window -> Network Traffic

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7819