GPencil: Include new Brush random curves

Now the brushes have several new random settings and use curves to define the effect. The curves have been moved below the parameter to keep UI standards and extra curve panels have been removed.

{F8505387}

The new curves are:

* Hue.
* Saturation.
* Value.

New option to random at stroke level instead to random at point level for the following values:

* Thickness.
* Strength.
* UV.
* Hue.
* Saturation.
* Value.

Curves have been moved below the corresponding parameter and only are displayed in properties panel. Display the curves in the popover made it unusable.

{F8505392}

Also, the Pressure random has been renamed to Radius because the old name was not clear enough.

Reviewed By: mendio, pablovazquez

Differential Revision: https://developer.blender.org/D7577

source/blender/draw/intern/draw_cache_impl_gpencil.c

@@ -544,7 +544,7 @@ static void gpencil_sbuffer_stroke_ensure(bGPdata *gpd, bool do_stroke, bool do_
       ED_gpencil_tpoint_to_point(region, origin, &tpoints[i], &gps->points[i]);
       mul_m4_v3(ob->imat, &gps->points[i].x);
       bGPDspoint *pt = &gps->points[i];
-      copy_v4_v4(pt->vert_color, gpd->runtime.vert_color);
+      copy_v4_v4(pt->vert_color, tpoints[i].vert_color);
     }
     /* Calc uv data along the stroke. */
     BKE_gpencil_stroke_uv_update(gps);