Improve Code quality about thorvg

[JSUYA]

Apart from method's complexity, some of our codes have an ordering dependency on header include. (Transitive inclusions)
The google c++ style guide provides some concepts to solve this problem.
https://google.github.io/styleguide/cppguide.html#Include_What_You_Use
https://google.github.io/styleguide/cppguide.html#Names_and_Order_of_Includes

what do you think about it?

[hermet]

@JSUYA Conceptually, each file should explicitly include headers if they depend on them. The problem is that somebody might forget to include a header, and it's not easy to detect this omission during the review stage because even reviewers may not notice it when transitive inclusions implicitly hide it. In my case, I'm not overly concerned about this, as any breakage can be promptly detected during testing. We can easily identify the missing header in the necessary file, allowing us to include it at that time.

The only header that truly matters for the transition is the client API header (thorvg.h/thorvg_capi.h) for us. And I believe it's well maintained.