vcd: Fix output for more than 8 channels.

(receive): Use probe index for sample byte selection too, not just
for bit selection.  Also simplify the indexing expressions a bit.
This fixes the problem of incorrect output for probes indices 8 to
31.
Also, use double rather than float in the timestamp calculation,
and format the result directly as floating point number rather than
converting it back to uint64_t.
Additionally, make sure that the state of all signals is written
for the very first sample in the stream.  This fixes the problem
that signals would be displayed as indeterminate until the first
change.
(context.samplecount): Make the sample counter part of the context
struct, rather than keeping it as a global static.