Synfig Animation Studio

This is a basic patch that adds the required code and the detection of
libswscale. It also keeps a fallback to the original code. There are four
points of discussion though:

* I used a pair of sws_getContext/sws_freeContext because I'm not clear on
the multi-threading aspect. Otherwise the more efficient
sws_getCachedContext could probably be used.

* I've taken the SWS_BICUBIC interpolation from the example link, but a
simpeler interpolation computation might be sufficient.

* On my system the fallback (deprecated function img_convert) is detected,
and the code (still) compiles, but the code fails (apparently somewhere
else before my changes are touched).

* The image output looks okay, but I've not looked at it in detail.