Bump rendering edges to fill axes when rendering affine non-NN interpolation

This fixes a visual issue where there can appear to be a slight gap
between an image and the axes spines even when it is supposed to fill
the axes.  The code now expands the rendering edges to fill the axes
without degrading the placement accuracy within the image.

Author: ayshih

src/_image_resample.h

@@ -759,13 +759,38 @@ void resample(
 
     agg::path_storage path;
     if (params.is_affine && params.interpolation != NEAREST) {
-        path.move_to(0, 0);
-        path.line_to(in_width, 0);
-        path.line_to(in_width, in_height);
-        path.line_to(0, in_height);
-        path.close_polygon();
-        agg::conv_transform<agg::path_storage> rectangle(path, params.affine);
-        rasterizer.add_path(rectangle);
+        if (params.affine.shx != 0 || params.affine.shy != 0) {
+            path.move_to(0, 0);
+            path.line_to(in_width, 0);
+            path.line_to(in_width, in_height);
+            path.line_to(0, in_height);
+            path.close_polygon();
+            agg::conv_transform<agg::path_storage> rectangle(path, params.affine);
+            rasterizer.add_path(rectangle);
+        } else {
+            // If there is no shear/rotation, bump out the rendering edges that are
+            // within a half pixel of a full pixel so that axes are visually filled.
+            // This bumping out is equivalent to treating any edge pixel that is at
+            // least half-covered by the source as fully covered by the source.
+            double left = 0;
+            double right = in_width;
+            double bottom = 0;
+            double top = in_height;
+            params.affine.transform(&left, &bottom);
+            params.affine.transform(&right, &top);
+            if (left > right) { std::swap(left, right); }
+            if (bottom > top) { std::swap(top, bottom); }
+            if (round(left) < left) { left = round(left); }
+            if (round(right) > right) { right = round(right); }
+            if (round(bottom) < bottom) { bottom = round(bottom); }
+            if (round(top) > top) { top = round(top); }
+            path.move_to(left, bottom);
+            path.line_to(right, bottom);
+            path.line_to(right, top);
+            path.line_to(left, top);
+            path.close_polygon();
+            rasterizer.add_path(path);
+        }
     } else {
         path.move_to(0, 0);
         path.line_to(out_width, 0);