Add OBS_SOURCE_SRGB to indicate sources that support SRGB rendering. We
can use this flag to know which sources do not know how to handle SRGB,
and disable accordingly inside obs_source_main_render().
We can also use this flag to clean up the filter interface and remove
the SRGB-specific functions. We also need to disable direct rendering if
the filter source wants to render SRGB, but the parent source does not
support it.
Scenes and groups are marked as having SRGB support, and those are
internal sources that we control.
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