#4 Improve implementation of constructors

[Markus Elfring]

I have noticed that some assignments are used in the constructor bodies.

Examples:
https://github.com/dov/giv/blob/ff10d17d42aabfd2edbd23c078363245af86a873/src/GivImage.cc#L14
https://github.com/dov/giv/blob/ff10d17d42aabfd2edbd23c078363245af86a873/src/GivPainterAgg.cc#L137

The recommended way for performing efficient construction is to use the initialisation list.
http://dietmar-kuehl.de/mirror/c++-faq/ctors.html#faq-10.6
http://cprogramming.com/tutorial/initialization-lists-c++.html
http://www.codeguru.com/cpp/tic/tic0145.shtml
http://www.informit.com/guides/content.aspx?g=cplusplus&seqNum=15&rl=1
http://goingware.com/tips/parameters/membervars.html