Do not support GCC 9 (#1978)

This will let us use C++20 features that GCC 9's experimental
C++20 support did not yet cover, such as "concepts".

This reverts some commits:
- 6bcd79b997
- d5ce5329ea
- 728d14879b

src/asm/fixpoint.cpp

@@ -5,10 +5,10 @@
 #include "asm/fixpoint.hpp"
 
 #include <math.h>
+#include <numbers>
 #include <stdint.h>
 
-// Ideally we'd use `std::numbers::pi`, but GCC 9 doesn't support it.
-static constexpr double tau = 3.141592653589793238462643383279502884L * 2;
+static constexpr double tau = std::numbers::pi * 2;
 
 static double fix2double(int32_t i, int32_t q) {
 	return i / pow(2.0, q);

src/gfx/pal_packing.cpp

@@ -100,7 +100,6 @@ private:
 		AssignedSetsIter() = default;
 
 		bool operator==(AssignedSetsIter const &rhs) const { return _iter == rhs._iter; }
-		bool operator!=(AssignedSetsIter const &rhs) const { return !operator==(rhs); }
 
 		AssignedSetsIter &operator++() {
 			++_iter;

src/gfx/process.cpp

@@ -263,7 +263,6 @@ struct Image {
 			}
 
 			bool operator==(Iterator const &rhs) const { return coords() == rhs.coords(); }
-			bool operator!=(Iterator const &rhs) const { return !operator==(rhs); }
 		};
 
 	public: