Fix rendering of multicharacter backgrounds

Author: urkerab

charcoal.py

@@ -601,16 +601,15 @@ def __str__(self):
         left = min(self.indices)
         right = max(self.right_indices)
         string = ""
-        bg_start = None
         if self.bg_lines:
             for i in range(len(self.lines)):
                 top = self.top + i
                 index = self.indices[i]
                 line = self.lines[i]
-                j = -1
+                bg_start = None
+                j = 0
                 if self.background_inside:
                     for character in line:
-                        j += 1
                         if character == "\000":
                             if bg_start is None:
                                 bg_start = j
@@ -623,6 +622,7 @@ def __str__(self):
                                 line[j:]
                             )
                             bg_start = None
+                        j += 1
                     if bg_start is not None:
                         line = (
                             line[:bg_start] +

test.py

@@ -289,6 +289,7 @@ def test_background(self):
 *#-#*
 *****""")
         self.assertEqual(Run("Ｆ⁶«Ｍ→_»ＵＢ| "), "_|_|_|_|_|_")
+        self.assertEqual(Run("ＵＢ#@↗³⟲"), "/@#\n#/#\n#@/")
         # TODO: weird, why does Charcoal escape asterisks???
         self.assertEqual(
             Run("Multiprint(:+, 'abc');SetBackground('*')", verbose=True),