media: ffmpeg: allow detection of encoder codecs
media: ffmpeg: allow detection of encoder codecs

Since we are going to support encoding, add a method to detect whether a
given codec is an encoder.

BUG=b:239897269
TEST=cargo build --features "video-decoder,ffmpeg"

Change-Id: I547fa6acafc4580f3f2f05c1a82955212d4d6e84
Reviewed-on: https://chromium-review.googlesource.com/c/crosvm/crosvm/+/3782041
Reviewed-by: Keiichi Watanabe <815e4e8f7c92a23424d7c3f51ead622bcedbc720@chromium.org>
Commit-Queue: Alexandre Courbot <301cd6a3ea50a894b4b51e385c5eb12b9a4ffc0c@chromium.org>
Tested-by: Alexandre Courbot <301cd6a3ea50a894b4b51e385c5eb12b9a4ffc0c@chromium.org>
Auto-Submit: Alexandre Courbot <301cd6a3ea50a894b4b51e385c5eb12b9a4ffc0c@chromium.org>

google/crosvm,google/crosvm,google/crosvm,google/crosvm