Stylo - support image-orientation property

First, we need to make Servo's image-orientation parser to be agreed with Gecko's.
Numbers without any AngleUnit, including unitless 0 angle, should be invalid
for image-orientation. However, rotate() and skew() for transform properties
accept unitless 0 angle. In order to make all these properties work properly,
I fixed Angle::parse() to match Gecko. For the existing users of Angle::parse(),
I create Angle::parse_with_unitless() and use it as an alternative for them.
Once https://github.com/w3c/csswg-drafts/issues/1162 is resolved, we shall be
able to use an unified version of Angle::parse() then.

The parser of image-orientation is also fixed to report parsing errors on
empty string.

Then, with the newly added binding functions support in Gecko side, we shall
reuse the same methods from Gecko to pass the computed value from Servo to Gecko.

Gecko bug: Bug 1341758

tests/unit/style/parsing/inherited_box.rs

@@ -17,7 +17,7 @@ fn image_orientation_longhand_should_parse_properly() {
     assert_eq!(from_image, SpecifiedValue { angle: None, flipped: false });
 
     let flip = parse_longhand!(image_orientation, "flip");
-    assert_eq!(flip, SpecifiedValue { angle: None, flipped: true });
+    assert_eq!(flip, SpecifiedValue { angle: Some(Angle::from_degrees(0.0)), flipped: true });
 
     let zero = parse_longhand!(image_orientation, "0deg");
     assert_eq!(zero, SpecifiedValue { angle: Some(Angle::from_degrees(0.0)), flipped: false });