Refactor swscale into new class + rename filtergraph and swscale constructs

[mollyxu]

Refactor sw_scale into new class similar to what we did for filtergraph. (#1135)

• Encapsulate the double swscale path (color conversion + resize) in a new SwScale class with a clean convert() API
• Merge SwScaleContext and SwsFrameContext into a single SwsConfig struct (colorspace is now part of the config rather than a separate parameter)
• Add encodingSwsContext_ for the encoding path, separating it from the decoding swscale which is now managed by SwScale

Rename filtergraph and swscale constructs in CpuDeviceInterface (#925)

[pytorch-bot]

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❌ 1 New Failure

As of commit 640efe4 with merge base cc15044 ():

NEW FAILURE - The following job has failed:

• Build and test Linux CUDA wheels and docs / install-and-test (3.10, 12.6, 4.4.2) (gh)
  Process completed with exit code 1.

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[NicolasHug]

Thanks @mollyxu !