FILTERS: Fixed bounds for filters frequency

Due to filters imprecision, this should be ok for now in order to use
automation & such without crashing the plugin.

Signed-off-by: Gama <misept.dieseneuf@gmail.com>
FILTERS: Fixed bad management of filter frequency normalization

This is now ok: the user sees it as a normalized value, and the filter
wrapper is in charge of de-normalization, leaving the dsp filter itself
as light as possible.

Signed-off-by: Gama <misept.dieseneuf@gmail.com>
SYNTH: Fixed various bad asserts due to size/capacity confusion

Sometimes this is the raw size_ variable member which is supposed to be
compared to - also sometimes a small margin is required.

Signed-off-by: Gama <misept.dieseneuf@gmail.com>
INTERPOLATOR: Now using a 2-samples long history

This is actually required for some ratio values.

Signed-off-by: Gama <misept.dieseneuf@gmail.com>
MISC: Minor style fixes

Signed-off-by: Gama <misept.dieseneuf@gmail.com>
TESTS: The synthesizer does no longer a minimal block size

Since it can now handle 1 sample long output, one previous test has been
simplified.

Signed-off-by: Gama <misept.dieseneuf@gmail.com>
TESTS: Added an interpolator test for a particular ratio value

Since 1.1 is a "special" value quite useful for testing purpose
(being non-integer, non-representable in 32b float standard), this special
case is being explicitly covered here.
TESTS: Added 2 tests for varying output format

These are testing:
- varying sampling rate with a fixed block size
- varying sampling rate with a varying block size

Signed-off-by: Gama <misept.dieseneuf@gmail.com>