perf(labels): compute np.unique(label) once per render

[timtreis]

What & why

Third item from the profiling write-up in #690 (after #691 shapes and #692 points).

_render_labels uniquified the (already-rasterized) label raster up to twice per render:

• once for the instance ids (np.unique(label)) or, when a table annotates the element,
  the overlap check (np.unique(label.values));
• again for the rasterize mask (labels_in_rasterized_image = np.unique(label.values)).

Both run on the same array, so the second pass is redundant.

Change

Compute the unique label values once after the (optional) rasterization, and reuse them
for the instance ids, the overlap check, and the rasterize mask.

Correctness — byte-identical output

Rendered 8 label scenarios on this branch and on main, comparing the Agg RGBA buffers
exactly (np.array_equal): all identical.

plain · contour · continuous · continuous fill+outline · categorical · groups ·
groups+na_color · outline-only

This covers both the no-table path (instance ids from the raster) and the
table-annotated path (overlap check + rasterize mask).

Performance

Removes one full-raster np.unique pass per render. The saving scales with the
displayed/rasterized label size (the label is downsampled to the canvas by default):

render	main	this PR
2000-px label, figsize 4×4, dpi 100	~289 ms	~284 ms (~5 ms)
2000-px label, figsize 12×12, dpi 300	~2336 ms	~2296 ms (~40 ms)

Small at default settings, larger for high-resolution / large-figure renders. It's a
correctness-neutral cleanup that also removes a redundant local variable.