Fix uninitialized HMD warping coefficient

The HMD warping shader uses the cubic warping coefficient, but we weren't
initializing it. This fix ensures it's set to zero in both HMD backends.

It's unsatisfying to force the GPU to perform a calculation using a value that
is known to be zero. We'd squeeze out a few cycles by eliminating the cubic
term from the shader. But it's there in the Oculus code, so we better keep it
here.

Oculus is *really* inconsistent in its handling of distortion coefficients.
The shader reads four, but the library writes three. Meanwhile, the device
itself reports six! The Oculus SDK avoids inconsistency by memsetting the
entire array to 0.

Props @parasti for uncovering this flaw.