Remove Justin.tv references/service

This removes all references to Justin.tv as the service is no longer in service.
Rename service from Twitch.tv to Twitch
Add reverse matrix multiply functions

The *_i functions multiply the value by the matrix instead of the other
way around, then store the result.
Fix gs_matrix_* issues

Multiplication of the matricies was being done in the wrong direction.
This caused source transformations to come out looking incorrect, for
example the linux-xshm source's cursor would not be drawn correctly or
in the right position if the source was moved/scaled/rotated.  The
problem just turned out to be that the gs_matrix_* functions were
multiplying in the wrong direction.  Reverse the direction of
multiplication, and the problem is solved.
Fix freetype cmake module

On fedora, the freetype cmake module would not find the correct
location.  This is because it wasn't using the pkg-cofig include
directory, which is different from the main ft2build.h header directory
(which is /usr/include).

This modification may be a good thing to do on all the cmake packages,
actually.  Currently, they all just search for the location of a
specific header, and ignore the pkg-config includes if they find it in
specific hint locations.
Remove Justin.tv references/service
Merge pull request #253 from drekryan/patch-1

Remove Justin.tv references/service
Fix null pointer dereference

When looking up translated font names within font files, it was not
checking for null on the 'charset' variable (the translation character
set wasn't added/availble).
Merge branch 'master' of https://github.com/jp9000/obs-studio