common paint: introduce opacity() method.
We introduced separate opacity interface to adjust alpha value by paint.
This opacity will affect to whole paint image if paint is a group of paints.
Also, this opacity is to multipy with fill/stroke alpha values.
This means if the opacity is valid, the paint might deal with a composition step,
which is very expensive due to additional rendering step.
One tip is, if you want to toggle on/off for a certian paint,
you can set opacity to 255 or 0.
@API Additions:
Result Paint::opacity(uint8_t o) noexcept;
uint8_t Paint::opacity() const noexcept;
@Examples: examples/Opacity
@Issues: 94
Change-Id: Ie291890304230c34006fbe7f96a8707e01d9439c
14 files changed: