Respect MAGICK_HOME on Windows

 - Don't read Windows registry if MAGICK_HOME was set, keep the original value
   of the environment variable.
 - It now seems to be enough to only add the path to the main DLL to PATH.
   This is also what the ImageMagick installer does if you ask it to add
   ImageMagick to PATH.

wand/api.py

@@ -46,22 +46,17 @@ def library_paths():
     magick_home = os.environ.get('MAGICK_HOME')
     magick_suffix = os.environ.get('WAND_MAGICK_LIBRARY_SUFFIX')
 
-    if system == 'Windows':
-        # ImageMagick installers normally install coder and filter DLLs in
-        # subfolders, we need to add those folders to PATH, otherwise loading
-        # the DLL later will fail.
+    if system == 'Windows' and magick_home is None:
         try:
+            # Try to read ImageMagick location from registry.
+            # This is necessary if the user did not add the ImageMagick directory
+            # to PATH.
             with winreg.OpenKey(winreg.HKEY_LOCAL_MACHINE,
                                 r"SOFTWARE\ImageMagick\Current") as reg_key:
                 libPath = winreg.QueryValueEx(reg_key, "LibPath")
-                coderPath = winreg.QueryValueEx(reg_key, "CoderModulesPath")
-                filterPath = winreg.QueryValueEx(reg_key, "FilterModulesPath")
                 magick_home = libPath[0]
-                os.environ['PATH'] += str((';' + libPath[0] + ";" +
-                                          coderPath[0] + ";" + filterPath[0]))
+                os.environ['PATH'] += ';' + str(magick_home)
         except OSError:
-            # otherwise use MAGICK_HOME, and we assume the coder and
-            # filter DLLs are in the same directory
             pass
 
     def magick_path(path):