Add support for setting global image compression..

quality. Uses an IMAGE_COMPRESSION_QUALITY in the settings.py, which, if
set, will set a quality attribute in the image processing backends. Each
image processing backend then can use the quality attribute in its "save"
method.

This has been implemented in both Pillow and Wand backends. It has been
tested with image that support this setting (JPG) and images that do not
support it (GIF, PNG) and the behavior is consistent.

This is an implementation for #163.

wagtail/wagtailimages/backends/base.py

@@ -1,4 +1,5 @@
 from django.db import models
+from django.conf import settings
 from django.core.exceptions import ImproperlyConfigured
 
 class InvalidImageBackendError(ImproperlyConfigured):
@@ -6,8 +7,13 @@ class InvalidImageBackendError(ImproperlyConfigured):
 
 
 class BaseImageBackend(object):
+    quality = None
+    
     def __init__(self, params):
-        pass
+        try:
+            self.quality = settings.IMAGE_COMPRESSION_QUALITY
+        except:
+            pass
         
     def open_image(self, input_file):
         """

wagtail/wagtailimages/backends/pillow.py

@@ -14,7 +14,10 @@ class PillowBackend(BaseImageBackend):
         return image
         
     def save_image(self, image, output, format):
-        image.save(output, format)
+        if self.quality:
+            image.save(output, format, quality = self.quality)
+        else:
+            image.save(output, format)
     
         
     def resize(self, image, size):

wagtail/wagtailimages/backends/wand.py

@@ -16,6 +16,8 @@ class WandBackend(BaseImageBackend):
         
     def save_image(self, image, output, format):
         image.format = format
+        if self.quality:
+            image.compression_quality = self.quality
         image.save(file=output)
         
     def resize(self, image, size):

wagtail/wagtailimages/models.py

@@ -193,7 +193,7 @@ class Filter(models.Model):
         input_file.open('rb')
         image = backend.open_image(input_file)
         file_format = image.format
-
+        
         method = getattr(backend, self.method_name)
 
         image = method(image, self.method_arg)
@@ -201,6 +201,9 @@ class Filter(models.Model):
         output = StringIO.StringIO()
         backend.save_image(image, output, file_format)
         
+        # and then close the input file
+        input_file.close()
+        
 
         # generate new filename derived from old one, inserting the filter spec string before the extension
         input_filename_parts = os.path.basename(input_file.name).split('.')