screencastService: Add rate control to VA-API H.264 pipelines
The vah264enc and vah264lpenc pipelines do not specify any rate control
parameters, causing the encoder to use its default bitrate. At 4K this
results in ~12 Mbit/s recordings — roughly 18x larger than the VP8
software fallback (~630 kbit/s) with no visible quality benefit for
typical screencast content.
All other pipelines in the PIPELINES array constrain quality explicitly:
openh264enc uses qp-max=26/qp-min=26, vp8enc uses max-quantizer=17 and
buffer-size=20000. Only the two VA-API pipelines are left unconstrained.
Add rate-control=vbr bitrate=2048 target-percentage=50 to both VA-API
pipelines. This produces ~1 Mbit/s average / 2 Mbit/s peak, which is
comparable to what the other encoders produce and works across both AMD
(which lacks CQP) and Intel VA-API implementations.
Before (4K, 5s recording): 11688 kb/s, 7.2 MB
After (4K, 5s recording): 2472 kb/s, 1.6 MB
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