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

include/gfx/rgba.hpp

@@ -40,7 +40,6 @@ struct Rgba {
 	}
 
 	bool operator==(Rgba const &rhs) const { return toCSS() == rhs.toCSS(); }
-	bool operator!=(Rgba const &rhs) const { return !operator==(rhs); }
 
 	// CGB colors are RGB555, so we use bit 15 to signify that the color is transparent instead
 	// Since the rest of the bits don't matter then, we return 0x8000 exactly.