obs-webrtc: Add "bframes" key override for mac VT encoder

[kc5nra]

Most of the encoders use "bf" as a key to store the number of bframes but VT and QSV use "bframes". QSV already has code to work around this issue.

NOTE There is another way to accomplish this, similar to the QSV encoder:

if (obs_data_has_user_value(settings, "bf"))
		bFrames = (int)obs_data_get_int(settings, "bf");

This seemed a bit "hidden behavior..y", but happy to do either way! Certainly would make it most consistent to override bframes in the future if everyone didn't need to reason about both bframes and bf.

Description

Adds "bframes" override in addition to "bf" to convert VT encoders.

Motivation and Context

The WHIP service overrides bframes using "bf" but the VT encoder uses "bframes" as the key. As a result, the encoder ends up sending bframes if they are configured in the output settings.

How Has This Been Tested?

Tested on Mac with and without keyframes specified in the output.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.