win-capture: Initialize window capture immediately

[prgmitchell]

Description

Initialize new window capture instances through the existing update path so the first lookup is not delayed by 1 second and instead happens on the next video tick.

NOTE: I did consider that using the OBS_SOURCE_DO_NOT_DUPLICATE flag for window capture sources would also fix but I am not sure if there is a specific reason we allow duplication for window capture sources but not display capture.

Motivation and Context

Fixes #4878

How Has This Been Tested?

Created two scenes with the same window capture source, enabled studio mode and the duplicate sources option, and then pressed the transition button to switch between the scenes. On the current release the program scene goes black for 1 second after pressing transition, with these changes it does not.

Types of changes

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

Checklist:

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

[PatTheMav]

Seems fine code-wise, might require some additional behavioural testing on Windows by other maintainers.