Manually force keyframes in encoder loop

[seanavery]

Description

Despite having successful 24 hr runs on linux, I am still getting reports of flaky behavior on Darwin. This PR removes FFmpeg handling of keyframe forces entirely, and instead adds in manual keyframe forces in the encoder loop.

• Removes gop_size encoder setter entirely to avoid any conflicting behavior.
• Removes keyframe force expression.
• Adds manual keyframe flag every second.

Testing

• linux ✅
  • 3 hour test, 500 segments ✅
  • overnight test ✅
• darwin ✅
  • 3 hour test, 500 segments ✅
  • overnight test ✅

[hexbabe]

Got it thanks for update