Add CodecContext global_quality property binding

[ladaapp]

Was trying to use QSV encoders but noticed that the property they use for quality-based mode is unavailable. This PR exposes it in PyAV.

Couldn't think of a useful test for this as QSV wouldn't be available in CI and i'm not aware of another video encoder that uses this (tested with libx264 but it's ignored).

Reference:

• https://ffmpeg.org/doxygen/0.8/structAVCodecContext.html#209f5ec60cb5f0b0a4962f4c5c5bb541
• https://ffmpeg.org/ffmpeg-codecs.html#QSV-Encoders
• https://trac.ffmpeg.org/wiki/Hardware/QuickSync
• https://github.com/mltframework/mlt/blob/c5534c8fa856940a397a1fe84d5ef3110d35ec43/src/modules/avformat/consumer_avformat.c#L1032

Example

import av
import av.datasets

input_container = av.open(av.datasets.curated("pexels/time-lapse-video-of-night-sky-857195.mp4"))
output_container = av.open("hevc_qsv_encoded.mp4", "w", options={})

input_stream = input_container.streams.video[0]
output_stream = output_container.add_stream('hevc_qsv', input_stream.average_rate)
output_stream.width = input_stream.width
output_stream.height = input_stream.height

output_stream.pix_fmt = "nv12"
output_stream.codec_context.global_quality = 20
output_stream.codec_context.qscale = True

for frame in input_container.decode(video=0):
    packet = output_stream.encode(frame)
    if packet: output_container.mux(packet)

input_container.close()
output_container.close()

[ladaapp]

Looks like this change is not needed and setting it like it's done for other video encoders does actually work (via Output Stream options dict)

output_stream.options = {"global_quality": "40"}

ffmpeg -h encoder=hevc_qsv doesn't mention it as an option but it still seems to work just fine.

@WyattBlue Feel free to close this MR if you think there is no need for it