Blender Git Loki

Fix / cleanup panel category tab drawing

The panel category tabs (In the 3D view N-panel) did not look great
when zoomed in or with high DPI screens, with multiple overlapping
outlines, and roundness that didn't match elsewhere in Blender. In
addition, there was some unecessarily low-level drawing code in the
panel code.

This commit uses an existing function `UI_draw_roundbox_4fv`to draw
the tabs instead. There are some slight visual differences, though
these are easily adjusted with theme colors.

Differential Revision: https://developer.blender.org/D9045