Bug 913619 - Minimize the #includes in image/src; r=jrmuizel

This patch was generated by running include-what-you-use on image/src,
and then removing the #include statements suggested by that tool, either
replacing them with forward declarations of the used names in headers,
or dropping the ones that were completely unnecessary, and then adding
new #include statements in other places that were implicitly relying on
some of the removed #include statements.