MixerToy: do an initialisation run on the mixer

The Interpolating mixer has internal state which must be setup by a
previous frame (we assume 0 for the first frame). Since we work on
exactly a period of the wave here, sending the same data twice works as
it should.
Interpolating resampler: un-break it.

The interpolation was performed only on the samples in a buffer,
without "linking" with the previous one. This can't work.
* Remember the last sample from a buffer to be able to use it when
interpolating the first samples of the next one
* Adjust the kernel to properly loop over all samples in the buffer

Fixes #9438.