[effects/mousemark] Properly use GL_LINE_SMOOTH

Summary:
We need to enable GL_BLEND for line smoothing to work.

CCBUG: 337044

Test Plan:
Before:
{F6184072, layout=center, size=full}

After:
{F6184074, layout=center, size=full}

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: davidedmundson, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D14714