Fix streaming to multiple clients from hardware encoder on Windows

[cgutman]

Description

#668 broke the multi-client streaming scenario because it assumes a single img_t is shared between a single capture device and a single encoding device. When using multiple clients, there is a new hwdevice_t for each independent stream, so there can be an arbitrary number of encoding devices sharing a single capture texture. This PR fixes this issue by keeping a map of encoder image context structures in the hwdevice_t, so each separate encoding device can open the same capture texture and mutex properly.

Screenshot

Issues Fixed or Closed

Fixes #795

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

Branch Updates

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must be updated before it can be merged. You must also
Allow edits from maintainers.

• I want maintainers to keep my branch updated

[cathyjf]

I've tested this pull request and it solves the problem for me. Thanks for fixing this bug!