Reduce grid rows if larger than number of images available

When a set number of grid rows is specified in settings, then it leads
to situations where an entire row in the grid is empty.
The most noticable example is the processing preview when the row count
is set to 2, where it shows the preview just fine but with a black
rectangle under it.

modules/images.py

@@ -36,6 +36,8 @@ def image_grid(imgs, batch_size=1, rows=None):
         else:
             rows = math.sqrt(len(imgs))
             rows = round(rows)
+    if rows > len(imgs):
+        rows = len(imgs)
 
     cols = math.ceil(len(imgs) / rows)