Requests: GetStreamScreenshot

[ghostzero]

Description

Adds a new request called GetStreamScreenshot which returns a Base64-encoded screenshot of the stream (program).

Motivation and Context

I require this feature for a remote production where I cannot use remote desktop software to see the actual program in real time. Currently there is no way to take a screenshot of the program, which is "immutable" until the transition button is pressed. This feature allows you to take a screenshot of the main texture frame (which also contains stinger transitions). This PR also closes #1130.

https://github.com/obsproject/obs-websocket/assets/6547306/e28a7131-cca6-4fa0-9525-538d417e9759

How Has This Been Tested?

Tested OS(s): Windows 11 22H2 22621.2715

I checked out the obs-studio repository, followed the build instructions, made changes to the plugins\obs-websocket folder, and built obs-studio to test my changes. Since this is mostly copy/paste from the source screenshot request with tweaks to capture the main texture, I expect no problems.

Types of changes

• New request/event (non-breaking)

Checklist:

• [x] I have read the Contributing Guidelines.
• [x] All commit messages are properly formatted and commits squashed where appropriate.
• [x] My code is not on master or a release/* branch.
• [x] The code has been tested.
• [x] I have included updates to all appropriate documentation.