output/vcd: assume packed input data image, unbreak 2nd+ channel

output/vcd: assume packed input data image, unbreak 2nd+ channel

The previous implementation only emitted data for the first enabled
channels, and "saw no changes" after emission of the initial value for
channels on positions that followed a disabled channel.

Assume that the received data from the session bus communicates the bits
of enabled channels in a packed representation. Skip the mapping of
output bit indices to sigrok channel numbers.

This fixes the remaining part of bug #519.

Tested by inspecting in gtkwave the result of command:
  $ sigrok-cli -d demo -C D1,D3,D6 -c samplerate=2M --samples 2500 -O vcd -o trace.vcd

When we find that all input sources (device drivers, and input modules)
provide a dense bit field, all of the mapping logic can get removed
here. This commit just quickly disables the logic.