Summary:
Based on the work of 3f4995fb9b this change
introduces a GlxContextAttributeBuilder to make the requesting of context
attributes cleaner, more verbose and less error prone copy and paste.
Test Plan:
Switched between Core and legacy and verified the output;
extended auto test
Reviewers: #kwin, #plasma
Subscribers: plasma-devel
Tags: #plasma
Differential Revision: https://phabricator.kde.org/D6411
