Fix behavior of volume envelope delay phase

[derselbst]

Previously, fluidsynth used an optimization that allowed silent voices to be completely skipped rendering. While this behavior might make sense from a performance point of view, it does break the expected behavior of the delay phase demanded by the SF2, see #1312 for details. Essentially, the delay phase shall not postpone the sample, rather the sample should be played "silently". And that's what this PR does. However, as the rendered samples will always become zero, the nearest neighbor interpolation is used as a minor optimization in this case.

Fixes #1312.