Remove blanket rasterization skip for images under 2000px

The guard `if x_dims < 2000 and y_dims < 2000: do_rasterization = False`
silently ignored the user's DPI intent — a low-DPI figure should still
trigger downsampling when the source image exceeds the target resolution.

Also re-adds the visual test for #310: at dpi=15 the 512x512 blobs image
is now correctly downsampled to ~96x72, producing visibly pixelated output
that would fail baseline comparison if DPI were overridden to the default.

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

Author: timtreis

src/spatialdata_plot/pl/utils.py

@@ -1960,10 +1960,10 @@ def _rasterize_if_necessary(
     target_y_dims = dpi * height
     target_x_dims = dpi * width
 
-    # Heuristics for when to rasterize
+    # Rasterize when the source image is substantially larger than what the
+    # current figure DPI × size requires.  The +100 margin avoids rasterizing
+    # when the image is only slightly larger than the target.
     do_rasterization = y_dims > target_y_dims + 100 or x_dims > target_x_dims + 100
-    if x_dims < 2000 and y_dims < 2000:
-        do_rasterization = False
 
     if do_rasterization:
         logger.info("Rasterizing image for faster rendering.")

tests/pl/test_show.py

@@ -42,6 +42,16 @@ def test_plot_no_decorations(self, sdata_blobs: SpatialData):
         """Visual test: frameon=False + title='' produces just the plot content (regression for #204)."""
         sdata_blobs.pl.render_images(element="blobs_image").pl.show(frameon=False, title="", colorbar=False)
 
+    def test_plot_user_ax_dpi_preserved(self, sdata_blobs: SpatialData):
+        """Visual test: low DPI produces visibly pixelated rasterization (regression for #310).
+
+        Uses dpi=15 so the 512x512 blobs image is downsampled to ~96x72.
+        If the bug regresses and DPI is overridden to the default (~100),
+        no rasterization occurs and the sharper render fails comparison.
+        """
+        fig, ax = plt.subplots(dpi=15)
+        sdata_blobs.pl.render_images(element="blobs_image").pl.show(ax=ax)
+
     def test_no_plt_show_when_ax_provided(self, sdata_blobs: SpatialData):
         """plt.show() must not be called when the user supplies ax= (regression for #362)."""
         _, ax = plt.subplots()