add default quality to imwrite and add comments

(cherry picked from commit 537b394)

airtest/aircv/aircv.py

@@ -26,7 +26,7 @@ def imread(filename, flatten=False):
     return img
 
 
-def imwrite(filename, img, quality):
+def imwrite(filename, img, quality=10):
     """写出图片到本地路径，压缩"""
     if PY2:
         filename = filename.encode(sys.getfilesystemencoding())

airtest/core/android/android.py

@@ -221,6 +221,7 @@ def snapshot(self, filename=None, ensure_orientation=True, quality=10):
         Args:
             filename: name of the file where to store the screenshot, default is None which is stdout
             ensure_orientation: True or False whether to keep the orientation same as display
+            quality: The image quality, integer in range [1, 99]
 
         Returns:
             screenshot output

airtest/core/ios/ios.py

@@ -163,6 +163,7 @@ def snapshot(self, filename=None, strType=False, quality=10):
         """
         take snapshot
         filename: save screenshot to filename
+        quality: The image quality, integer in range [1, 99]
         """
         data = None
 

airtest/core/win/win.py

@@ -105,6 +105,7 @@ def snapshot(self, filename=None, quality=10):
 
         Args:
             filename: name of the file to give to the screenshot, {time}.jpg by default
+            quality: The image quality, integer in range [1, 99]
 
         Returns:
             display the screenshot