vp8 decoder: cast decoding_thread_count to int
vp8 decoder: cast decoding_thread_count to int

For some reason allocated_decoding_thread_count is signed, but decoding_thread_count is not.

Cleans -Wextra/-Wsign-compare:
comparison between signed and unsigned integer expressions

Change-Id: Id0ada78100acff27c1c4ed7493c563d13c55cdcd

mwgoldsmith/vpx,Topopiccione/libvpx,mwgoldsmith/libvpx,mwgoldsmith/vpx,mwgoldsmith/libvpx,Topopiccione/libvpx,webmproject/libvpx,mwgoldsmith/vpx,Topopiccione/libvpx,ShiftMediaProject/libvpx,mwgoldsmith/vpx,mwgoldsmith/vpx,mwgoldsmith/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,Topopiccione/libvpx,mwgoldsmith/libvpx,mwgoldsmith/vpx,webmproject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,mwgoldsmith/libvpx,webmproject/libvpx,Topopiccione/libvpx,webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,mwgoldsmith/libvpx,Topopiccione/libvpx