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
15dda0115c