Add visual tests for transparent cmap with shapes rendering

Add test_plot_transparent_cmap_shapes_matplotlib and
test_plot_transparent_cmap_shapes_datashader to verify that shapes with
value=0 are rendered transparently over an image when using
set_zero_in_cmap_to_transparent, for both rendering backends.

Placeholder baselines included — CI will generate the real ones.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: timtreis

tests/_images/Utils_transparent_cmap_shapes_datashader.png

@@ -58,8 +58,6 @@ def test_plot_colnames_that_are_valid_matplotlib_greyscale_colors_are_not_evalua
         sdata_blobs.pl.render_shapes("blobs_polygons", color=colname).pl.show()
 
     def test_plot_can_set_zero_in_cmap_to_transparent(self, sdata_blobs: SpatialData):
-        from spatialdata_plot.pl.utils import set_zero_in_cmap_to_transparent
-
         # set up figure and modify the data to add 0s
         _, axs = plt.subplots(nrows=1, ncols=2, layout="tight")
         sdata_blobs.tables["table"].obs["my_var"] = list(range(len(sdata_blobs.tables["table"].obs)))
@@ -79,6 +77,21 @@ def test_plot_can_set_zero_in_cmap_to_transparent(self, sdata_blobs: SpatialData
             ax=axs[1], colorbar=False
         )
 
+    def _render_transparent_cmap_shapes(self, sdata_blobs: SpatialData, method: str):
+        new_cmap = set_zero_in_cmap_to_transparent(cmap="viridis")
+        sdata_blobs["table"].obs["region"] = pd.Categorical(["blobs_polygons"] * sdata_blobs["table"].n_obs)
+        sdata_blobs["table"].uns["spatialdata_attrs"]["region"] = "blobs_polygons"
+        sdata_blobs.shapes["blobs_polygons"]["value"] = [0.0, 2.0, 3.0, 4.0, 5.0]
+        sdata_blobs.pl.render_images("blobs_image").pl.render_shapes(
+            "blobs_polygons", color="value", cmap=new_cmap, method=method
+        ).pl.show(colorbar=False)
+
+    def test_plot_transparent_cmap_shapes_matplotlib(self, sdata_blobs: SpatialData):
+        self._render_transparent_cmap_shapes(sdata_blobs, method="matplotlib")
+
+    def test_plot_transparent_cmap_shapes_datashader(self, sdata_blobs: SpatialData):
+        self._render_transparent_cmap_shapes(sdata_blobs, method="datashader")
+
 
 @pytest.mark.parametrize(
     "color_result",