Fix image leak of hardware parallel encoder on display reinit

[psyke83]

Description

When a reinit caused by a capture size change occurs via the Windows hardware encoder, the stream will freeze with display_wp->use_count() stuck at 2. Fix by ensuring pending images are freed before reinit event is checked.

To reproduce:

• Using an application that allows full-screen transition via alt-enter, continually enter and exit fullscreen whilst the application is configured to use a different resolution from the desktop. In my case, I used Sonic Mania @ 1360x768 with a desktop resolution of 1440p, and the freeze usually occurs within 10 successive resolution changes.
• Disabling DwmFlush appears to help reproduce the issue more quickly.

The freeze will occur when this line appears in the log:

[2023:01:24:02:01:26]: Info: Capture size changed [1360x768 -> 2560x1440]

Further debugging shows that display_wp->use_count() stays at 2, causing Sunshine to freeze as it is stuck in this loop.

Pinging @cgutman, as it is related to: adbfd09. Please let me know if moving this breaks the original case it was intended to fix.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Dependency update (updates to dependencies)
• Documentation update (changes to documentation)
• Repository update (changes to repository files, e.g. .github/...)

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated the in code docstring/documentation-blocks for new or existing methods/components

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