Noticeable delay when using TAP Vibrato

* Operating system: 64-bit Arch Linux
* LMMS version: 1.2.0-rc5.9

When using the bundled TAP Vibrato effect the sound is consitently delayed by a split second.

### Steps to reproduce
1. Load any project
2. Add a TAP Vibrato effect with [the following settings](https://user-images.githubusercontent.com/6345473/35754518-bb2b5ed8-0831-11e8-82d1-fcfbff7708c1.png).

Expected behaviour: a vibrato is applied to the sound.

Actual behaviour: the vibrato is indeed applied, but accompanied with a delay.

Sample [project file](https://www.dropbox.com/s/i9n9p95h876ihmx/tap_vibrato.mmpz?dl=0), which renders to [this](https://www.dropbox.com/s/ulufbiphfco5dn8/tap_vibrato.ogg?dl=0).

Reproduce the delay in LMMS on 64-bit Arch Linux by loading the linked sample project, adding the bundled TAP Vibrato effect with the shown settings, and comparing its rendered output with the expected vibrato. Trace the TAP Vibrato audio-processing entry point to determine why the effect adds latency; done means the same reproduction steps produce vibrato without the audible split-second delay.

Written by the indexing model from the issue text.