Blender Git Loki

UI: Widget: Replace geometry by fragment shader drawing

This means all the antiailasing is done inside the fragment shader.

We use a Signed Distance Field to draw the 2D rounded boxes. This ensure
the best quality for AA.

This reduce the averge Batch for widget to 16 verts instead of ~600 and
reduce overshading a lot.

Theme Emboss alpha and tria alpha needs to be changed after this refactor.

The shadow drawing is left unchanged and still use geometry.

Reviewed By: Severin

Differential Revision: https://developer.blender.org/D7833