Resolves a GCC-troubling circular declaration issue vs. atomic.h.

Outputs/OpenGL/ScanTarget.hpp

@@ -75,6 +75,10 @@ class ScanTarget: public Outputs::Display::ScanTarget {
 		};
 
 		struct PointerSet {
+			// This constructor is here to appease GCC's interpretation of
+			// an ambiguity in the C++ standard; cf. https://stackoverflow.com/questions/17430377
+			PointerSet() {}
+
 			// The sizes below might be less hassle as something more natural like ints,
 			// but squeezing this struct into 64 bits makes the std::atomics more likely
 			// to be lock free; they are under LLVM x86-64.