AVX::Mask: fix operator==

AVX::Mask::operator== used _mm(256)_testc_ps to check for equality. This
works for many combinations but not if the left operand isEmpty() or the
right operand isFull(). The only correct solution seems to be an integer
compare of the movemask results of both operands.

closes #684

Signed-off-by: Matthias Kretz <kretz@kde.org>