patheffects.SimpleLineShadow calling non-existent get_foreground method from GraphicsContextBase (matplotlib#28814)

* Replaced get_forground() in patheffects.py with get_rgb()

* Built a unit test to test simpleLineShadow_plot

Author: anijjar

lib/matplotlib/patheffects.py

@@ -325,7 +325,7 @@ def draw_path(self, renderer, gc, tpath, affine, rgbFace):
         gc0.copy_properties(gc)
 
         if self._shadow_color is None:
-            r, g, b = (gc0.get_foreground() or (1., 1., 1.))[:3]
+            r, g, b = (gc0.get_rgb() or (1., 1., 1.))[:3]
             # Scale the colors by a factor to improve the shadow effect.
             shadow_rgbFace = (r * self._rho, g * self._rho, b * self._rho)
         else:

lib/matplotlib/tests/test_patheffects.py

@@ -2,7 +2,7 @@
 
 import numpy as np
 
-from matplotlib.testing.decorators import image_comparison
+from matplotlib.testing.decorators import image_comparison, check_figures_equal
 import matplotlib.pyplot as plt
 import matplotlib.patheffects as path_effects
 from matplotlib.path import Path
@@ -214,3 +214,19 @@ def close_group(self, s):
 
     assert renderer.open_group('s') == "open_group overridden"
     assert renderer.close_group('s') == "close_group overridden"
+
+
+@check_figures_equal()
+def test_simple_line_shadow(fig_test, fig_ref):
+    ax_ref = fig_ref.add_subplot()
+    ax_test = fig_test.add_subplot()
+
+    x = np.linspace(-5, 5, 500)
+    y = np.exp(-x**2)
+
+    line, = ax_test.plot(
+        x, y, linewidth=5,
+        path_effects=[
+            path_effects.SimpleLineShadow(offset=(0, 0), shadow_color='blue')])
+
+    ax_ref.plot(x, y, linewidth=5, color='blue', alpha=0.3)