perf(points): strip alpha on unique colours, not once per point (#692)

Author: timtreis

src/spatialdata_plot/pl/render.py

@@ -1341,7 +1341,11 @@ def _render_points(
             and isinstance(color_vector[0], str)
             and color_vector[0].startswith("#")
         ):
-            color_vector = np.asarray([_hex_no_alpha(c) for c in color_vector])
+            # color_vector usually holds only a few distinct hex strings (one per
+            # category), so strip alpha on the unique values and map back rather than
+            # calling the per-string parser once per point.
+            unique_hex, inverse = np.unique(color_vector, return_inverse=True)
+            color_vector = np.asarray([_hex_no_alpha(c) for c in unique_hex])[inverse]
 
         shade_how = render_params.density_how if render_params.density else "linear"
         # Plain density (no color column) must use the user-facing cmap as a sequential