Remove angled UI tabs

[Alexey]

I want, maybe, to remove angled kind of UI tabs. ie, make tabs always reactangled. so option ui_tab_angle will be removed.
reason:
angled tabs paint very slow [because of current code].
angled tabs paint with bug on macOS.
angled tabs give ugly code in ATTabs source.

[qwerky]

Hi Alexey,

I use angled tabs, but it is just for fun. It does not bother me at all if the angles are removed. Just one opinion.

[Alexey]

Maybe later I ll make new angled tabs. they have look with smooth edges and round edges. like in Firefox or Sublime.