GStreamer 1.0: Allocate element->h_snglsnr_buffer once instead of resizing

I've run into this in gstreamer_python_upgrade, though it could be fixed in py2.

Currently we set element->h_snglsnr_buffer based on the size of an incoming buffer in multiratespiir. If an incoming bufer is larger than the amount we've already allocated, we free and reallocate.

Its pointer is passed into spiirup, which does its computations and copies the results from device to that pointer. At the end, the entire buffer is copied onto our gstreamer buffer.

We could just allocate a size based on num_exe_samples (the size of a single buffer), and do multiple copies to the gstreamer buffer (in what I Believe is the rare case that an incoming buffer is larger than one execution size). I've done a quick test, and it doesn't seem to have a significant positive or negative performance impact, but prevent some crazy scenario from consuming all our memory.