fix: 1181 fix colorspace changes when session clear and fix memory leak with orphaned sessions

[mcoliver]

source_setup.py and ocio_source_setup.py scripts track whether a new Display Group has been created using a variable named _haveNewDisplayGroups. When you open OpenRV normally, this is configured correctly, and properties like sRGB output on the #RVDisplayColor node get activated. However, when you run File -> Clear, the C++ Session::clear() function destroys and immediately recreates the Display Group. The python scripts were failing to track this recreation because of missing event listeners.

When I loaded an MP4 file immediately after clearing the session, the C++ code successfully identified the MP4 as sRGB and enabled the RVLinearize step (darkening the image), but the python scripts skipped setting up the #RVDisplayColor node. Because #RVDisplayColor remained off, the linear image was never lifted back into display color space, rendering it way too dark.

I fixed this by adding an explicit listener for the after-clear-session event in both source_setup.py and ocio_source_setup.py. Now, immediately following a session clear, the internal Python flags are properly reset ensuring that the display color setup happens properly when the next file is loaded.

Also included a fix for a hardware memory leak. Every time you open an MP4 and then File -> Clear, OpenRV leaks a VideoToolbox hardware decoding session in macOS. Once you hit the macOS limit for concurrent hardware decoder sessions, FFmpeg will silently fail to allocate a new hardware decoder and fall back to the much slower software decoder (libx264). This fix frees up hardware decoding sessions so they can be available again.

Fixes #1181

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[bernie-laberge]

Two great catches ! Thank you @mcoliver !
We'll make sure to fast track that one.