[CSS] Support CSS Color Module Level 3

[ShikiOkasaka]

cf. http://www.w3.org/TR/css3-color/

• [x] the ‘opacity’ property
• [x] RGBA color values; ee152270e86542b4f96305b321d404614a17d3ec, 2d833d7b4b6653ee27267ebaaba3334ffa771768
• [x] HSL and HSLA color values; 0046a42db0f25df3e90eba1c0633bad41707681d
• [ ] ‘currentColor’ color keyword :arrow_right: #58

s.3.2.a should read: Since an element with opacity less than 1 is composited from a single offscreen image, content outside of it cannot be layered in z-order between pieces of content inside of it. For the same reason, implementations must create a new stacking context for any element with opacity less than 1. If an element with opacity less than 1 is not positioned, then it is painted on the same layer, within its parent stacking context, as positioned elements with stack level 0. If an element with opacity less than 1 is positioned, the ‘z-index’ property applies as described in [CSS21], except that if the used value is ‘auto’ then the element behaves exactly as if it were ‘0’. cf. http://www.w3.org/Style/2011/REC-css3-color-20110607-errata.html

cf. CSS3 Color Conformance Test Suite http://test.csswg.org/suites/css3-color/

[ShikiOkasaka]

TODO items for the ‘opacity’ property:

• [x] Add the 'opacity' property to CSSStyleDeclaration; 80dddfac5e7be7222069ae47593954fdb3c76773
• [x] Refine the Canvas class to support simple alpha compositing; d0a3b0e5328bd6019306ad1dd451fb39d687c5d2, eb04c9a66fa86b28e887514590918e7425e694da, 4dcc030a0870430f40aa1536e51c6ff7cede2360
• [x] Revise CSSStyleDeclarationImp::computeStackingContext to check 'opacity'; 00eaa70542acfbc9199252106515e479f6552aea
• [x] Switch to the translucent mode when rendering a stacking context created for an element with opacity less than 1; 2a9f4f07fcb2307a3f9c8145415306e421709d14

[ShikiOkasaka]

With 2a9f4f07fcb2307a3f9c8145415306e421709d14, the first three elements with opacity less than 1 in http://test.csswg.org/suites/css3-color/nightly-unstable/html4/t32-opacity-offscreen-with-alpha-c.htm are rendered correctly: The last one uses an RGBA color value, which is not supported yet.

[ShikiOkasaka]

RGBA color values are support at ee152270e86542b4f96305b321d404614a17d3ec:

http://test.csswg.org/suites/css3-color/nightly-unstable/html4/t32-opacity-offscreen-with-alpha-c.htm

[ShikiOkasaka]

Note now that we support RGBA color values, CSSSerializeRGB() does not have to return 'transparent'.

[ShikiOkasaka]

CSSSerializeRGB() is fixed at 68b1dae7275476e6bb5d2b74e009469f2ffba877.

[ShikiOkasaka]

The priority for supporting the ‘currentColor’ color keyword is not very high for escort at this point. We'll handle it in a separate issue #58.