Stop applying colorbar alpha twice (#687)

`fig.colorbar(mappable)` already bakes the mappable's alpha into the colorbar's
QuadMesh facecolors (verified: alpha column = mappable.alpha straight after
construction). Our subsequent `cb.solids.set_alpha(spec.alpha)` then multiplied
on top, so the colorbar rendered at alpha squared and looked much paler than the
layer it represents.

Only apply `spec.alpha` when the mappable does not carry alpha of its own; if it
does, trust the inherited value. Visible improvement on every continuous-color
colorbar (especially labels at low fill_alpha, e.g.
`Labels_can_handle_dropping_small_labels_after_rasterize_continuous` and
`Labels_two_calls_with_coloring_result_in_two_colorbars`).

Author: timtreis

src/spatialdata_plot/pl/basic.py

@@ -1637,7 +1637,11 @@ def _draw_colorbar(
             final_label = global_label_override or layer_label_override or spec.label
             if final_label:
                 cb.set_label(final_label)
-            if spec.alpha is not None:
+            # `fig.colorbar(mappable)` already bakes the mappable's alpha into the colorbar's
+            # facecolors, so we only apply `spec.alpha` when the mappable carries no alpha of its own
+            # — otherwise we'd multiply alpha twice and the colorbar would render at alpha squared
+            # (much paler than the layer it represents).
+            if spec.alpha is not None and spec.mappable.get_alpha() is None:
                 with contextlib.suppress(Exception):
                     cb.solids.set_alpha(spec.alpha)