UI: Fix Qt call on UI thread from graphics thread

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Calling `devicePixelRatioF` from any thread but the main UI thread
triggers thread-safety warnings at runtime on macOS, because Qt uses
NSView calls to determine the value.

NSView calls are only allowed to be made from the main thread on macOS,
so instead the value is stored as a property of the OBSQTDisplay at
initialization, to be retrieved from the preview object later.

Static functions that do not have access to the preview object have the
pixel ratio passed in their call signature.

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