UI: Round-box drawing cleanup

The new GPU_SHADER_2D_WIDGET_BASE shader allows us to draw
many complex shapes with anti-aliasing.
One thing it can do is draw an opaque rounded rectangle with colors
that differ between its interior and outline.

In order to do the above in a single pass I recently added an "_ex"
version of UI_draw_roundbox that exposes most of that shaders features.

This simplifies interface_draw.c by removing redundancy in the calling
of this shader by using this new uber "_ex" version.

Ref D10189

source/blender/editors/include/UI_interface.h

@@ -450,7 +450,7 @@ void UI_draw_roundbox_4fv_ex(float minx,
                              const float inner1[4],
                              const float inner2[4],
                              float shade_dir,
-                             float outline[4],
+                             const float outline[4],
                              float outline_width,
                              float rad);