Biquad: Highshelf filter

[rtoy]

A follow-up to #419, for a lowshelf filter

Blink uses

A = 10^(dbGain/40)
S = 1; // filter slope (1 is max value)
alpha = 0.5 * sin(w0) * sqrt((A + 1 / A) * (1 / S - 1) + 2);
k = cos(w0);
k2 = 2 * sqrt(A) * alpha;
aPlusOne = A + 1;
aMinusOne = A - 1;

b0 = A * (aPlusOne + aMinusOne * k + k2);
b1 = -2 * A * (aMinusOne + aPlusOne * k);
b2 = A * (aPlusOne + aMinusOne * k - k2);
a0 = aPlusOne - aMinusOne * k + k2;
a1 = 2 * (aMinusOne - aPlusOne * k);
a2 = aPlusOne - aMinusOne * k - k2;

This matches the cookbook except the Q parameter (called S above) is fixed to 1.