GIG Player Plugin

For a while I've wished to use the Maestro Concert Grand on linuxsampler.org in LMMS (#786). This piano is in the GIG file format, so I created a GIG player plugin (heavily based on the SF2 player plugin). The graphics aren't all that great and it doesn't support all of features of the format (there are quite a few; this plugin supports the features I could test), but it's a start. This plugin depends on libgig to get the data from the GIG file. Also, I haven't tested this on anything but Linux.

Previously the speed issues I was having were because I was loading the entire sample into memory at the beginning of each note. Now it loads the portions of the note as it renders each set of frames, which allows you to play many notes without glitching even at low latency (in my experience, at least).

Thoughts?
these conversion functions would be good to move into lmms_math.h as inline functions, in case we need them elsewhere...