compositor: perform conversions across all pads in parallel

commit 35bcd659

compositor: perform conversions in parallel

Improves throughput of the total convert and blend process and allows
for higher performance across slightly more threads.

Also make use of video aggregator's task pool for blending as well in
order to reduce the number of threads.

commit f325ccb2

video/aggregator: add parallel convert pad class

Each required conversion will be performed concurrently

commit 0550a795

video/converter: add support for async conversion operation

Allows for users to start up multiple conversions concurrently.