Fix: rerun check for imagemagick in isAvailable

On windows the check ensures that the imagemagick path for 'convert' was
set to '' if it was just 'convert' to prevent clashes with the windows
built-in command 'convert' (doing filesystem conversion). Unfortunately,
the command only set this values in the current rcParams and rcParamsDefault,
but a use of `matplotlib.style.use("classic")` after importing mpl.animation
would again overwrite it with `convert`.

In this case, the image comparison decorator sets the style to classic and
appveyor went BOOM...

Author: jankatins

lib/matplotlib/animation.py

@@ -338,10 +338,11 @@ def isAvailable(cls):
         Check to see if a MovieWriter subclass is actually available by
         running the commandline tool.
         '''
-        if not cls.bin_path():
+        bin_path = cls.bin_path()
+        if not bin_path:
             return False
         try:
-            p = subprocess.Popen(cls.bin_path(),
+            p = subprocess.Popen(bin_path,
                              shell=False,
                              stdout=subprocess.PIPE,
                              stderr=subprocess.PIPE,
@@ -642,12 +643,22 @@ def _init_from_registry(cls):
                 binpath = ''
         rcParams[cls.exec_key] = rcParamsDefault[cls.exec_key] = binpath
 
+    @classmethod
+    def isAvailable(cls):
+        '''
+        Check to see if a MovieWriter subclass is actually available by
+        running the commandline tool.
+        '''
+        bin_path = cls.bin_path()
+        if bin_path == "convert":
+            cls._init_from_registry()
+        return super(ImageMagickBase, cls).isAvailable()
 
 ImageMagickBase._init_from_registry()
 
 
 @writers.register('imagemagick')
-class ImageMagickWriter(MovieWriter, ImageMagickBase):
+class ImageMagickWriter(ImageMagickBase, MovieWriter, ):
     def _args(self):
         return ([self.bin_path(),
                  '-size', '%ix%i' % self.frame_size, '-depth', '8',
@@ -657,7 +668,7 @@ def _args(self):
 
 
 @writers.register('imagemagick_file')
-class ImageMagickFileWriter(FileMovieWriter, ImageMagickBase):
+class ImageMagickFileWriter(ImageMagickBase, FileMovieWriter):
     supported_formats = ['png', 'jpeg', 'ppm', 'tiff', 'sgi', 'bmp',
                          'pbm', 'raw', 'rgba']