The picture->size() function does not work for raw/png pictures.
As a consequence enabling both, the svg and png file loaders,
resulted in the unintended behavior of the example.
This implementation supports shape + stroke opacity composition.
Currently, tvg shape provides individual alpha values for filling & stroking
These alpha values are working individually, meaning that if stroking is half translucent,
user can see that translucent stroking is crossed the shape outlines.
Sometimes this result can be expected but user also expects the shape filling is invisible
behind of translucent stroking.
For this reason, Paint provides an additional api opacity()
that applies opacity value to whole paint attributes.
This is a little expensive job, please consider if you can possibly avoid that usage.
See Opacity example.
@Issues: 94
It missed to update shape data if visilibity is changed from false to true by alpha.
Also, it needs to update engine shape data for every requests.
There scenario can be allowed,
1. update shape
2. change shape property
3. update shape
4. draw
previously engine could skip step 3, its result was not properly expected.
@fix #84
