libweston: Optimise matrix multiplication

The previous version used div() to separate the column and row of the
current element, but that function is implemented as a libc call, which
prevented the compiler from vectorising the loop and made matrix
multiplication appear quite high in profiles.

With div() removed, we are down from 64 calls to vfmadd132ss acting on
one float at a time, to just 8 calls to vfmadd132ps when compiled with
AVX2 support (or 16 mulps, 16 addps with SSE2 support only), and the
function isn’t a hot spot any more.

Signed-off-by: Emmanuel Gil Peyrot <linkmauve@linkmauve.fr>