Implement dash pattern scaling in pdf

Factor the dash pattern scaling into a separate function so
GraphicsContextPdf.delta can call it. Combine the linewidth
and dash comparisons because now linewidth affects dashing.

Fixes matplotlib#6588

lib/matplotlib/backend_bases.py

@@ -858,6 +858,19 @@ def get_clip_path(self):
             return self._clippath.get_transformed_path_and_affine()
         return None, None
 
+    @staticmethod
+    def scale_dashes(linewidth, offset_dashes):
+        """
+        Scale the given offset, dashlist tuple to fit linewidth.
+        """
+        scale = max(1.0, linewidth)
+        offset, dashes = offset_dashes
+        if offset is not None:
+            offset = offset * scale
+        if dashes is not None:
+            dashes = [x * scale for x in dashes]
+        return offset, dashes
+
     def get_dashes(self):
         """
         Return the dash information as an offset dashlist tuple.
@@ -874,13 +887,7 @@ def get_dashes(self):
         if rcParams['_internal.classic_mode']:
             return self._dashes
         else:
-            scale = max(1.0, self.get_linewidth())
-            offset, dashes = self._dashes
-            if offset is not None:
-                offset = offset * scale
-            if dashes is not None:
-                dashes = [x * scale for x in dashes]
-            return offset, dashes
+            return self.scale_dashes(self.get_linewidth(), self._dashes)
 
     def get_forced_alpha(self):
         """

lib/matplotlib/backends/backend_pdf.py

@@ -2206,15 +2206,17 @@ def capstyle_cmd(self, style):
     def joinstyle_cmd(self, style):
         return [self.joinstyles[style], Op.setlinejoin]
 
-    def linewidth_cmd(self, width):
-        return [width, Op.setlinewidth]
-
-    def dash_cmd(self, dashes):
-        offset, dash = dashes
-        if dash is None:
-            dash = []
-            offset = 0
-        return [list(dash), offset, Op.setdash]
+    def linewidth_dash_cmd(self, width, dashes):
+        result = []
+        if self.get_linewidth() != width:
+            result += [width, Op.setlinewidth]
+        if self._dashes != dashes:
+            offset, dash = self.scale_dashes(width, dashes)
+            if offset is None or dash is None:
+                dash = []
+                offset = 0
+            result += [list(dash), offset, Op.setdash]
+        return result
 
     def alpha_cmd(self, alpha, forced, effective_alphas):
         name = self.file.alphaState(effective_alphas)
@@ -2288,14 +2290,11 @@ def clip_cmd(self, cliprect, clippath):
         (('_capstyle',), capstyle_cmd),
         (('_fillcolor',), fillcolor_cmd),
         (('_joinstyle',), joinstyle_cmd),
-        (('_linewidth',), linewidth_cmd),
-        (('_dashes',), dash_cmd),
+        (('_linewidth', '_dashes'), linewidth_dash_cmd),
         (('_rgb',), rgb_cmd),
         (('_hatch',), hatch_cmd),  # must come after fillcolor and rgb
         )
 
-    # TODO: _linestyle
-
     def delta(self, other):
         """
         Copy properties of other into self and return PDF commands