Attempt to fix threading issues that cause OBS to force-crash when
compiled with latest Xcode. There are two places where the new SDK
introduces a force-crash because operations are not happening on the
main thread: when we modify the context view to switch swap chains, and
when we resize a swap chain.
Instead of using just one context for all rendering, we create an
additional context for each swap chain, set each view once on
initialization, and switch contexts only in present to blit the final
framebuffer. This is an extra copy, but it's pretty hairy to optimize
away, and it's not worth potential regressions just to speed up Mac.
For resizing, we schedule the update code to run on the main thread from
the render thread. Ideally, we wouldn't have to round trip the logic
from main thread to graphics thread and back, but I don't think we want
to hack up the interface for this, especially since OpenGL will give way
to Metal soon enough.
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