input/vcd: fixup VCD timestamp to sigrok samplenum mapping

input/vcd: fixup VCD timestamp to sigrok samplenum mapping

When processing of large VCD input files was spread across multiple
parse_contents() invocations, the resulting sigrok stream of sample data
had gaps in them and total timing was off. For instance 74ms of input
data were interpreted as spanning some 600ms or 300ms, depending on the
number of channels in the input stream.

Move the "previous timestamp" variable to the input module context. This
eliminates the inappropriate gaps and fixes the translation of VCD file
timestamps to sigrok sample numbers.

This fixes bug #1075.