obs-outputs: Make MP4 output buffer/chunk size configurable

[derrod]

Description

Adds buffer_size/chunk_size options to configure the buffered file writers buffer size.

Motivation and Context

In some cases, e.g. high bitrate with high latency storage (NAS, SMR HDD) the default buffer size can be insufficient, and increasing it is relatively painless but not something we probably want to do by default.

How Has This Been Tested?

Compiles and still works.

Types of changes

• Tweak (non-breaking change to improve existing functionality)

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.