fix: Default to black fill color for shapes in animations

Previously, they were transparent unless an explicit color was set, which could be confusing, particularly since we opted not to show outlines.

Signed-off-by: George Thomas <[email protected]>

primer/src/Primer/Primitives.hs

@@ -47,12 +47,14 @@ import Diagrams.Backend.Rasterific (
 import Diagrams.Prelude (
   Diagram,
   V2 (..),
+  black,
   circle,
   deg,
   fillColor,
   lineWidth,
   mkP2,
   mkSizeSpec,
+  recommendFillColor,
   rect,
   rectEnvelope,
   renderDia,
@@ -402,7 +404,7 @@ primFunDef def args = case def of
       Con _ c [x] | c == cSucc -> succ <$> exprToNat x
       _ -> Nothing
     exprToDiagram e =
-      exprToPicture e <&> fix \f -> \case
+      exprToPicture e <&> recommendFillColor black . fix \f -> \case
         Circle r ->
           if r == 0 -- `diagrams` crashes with a divide-by-zero if we don't catch this case
             then mempty