Adds an interface allowing keyboard mappers to declare modifiers that are 'essential'.

i.e. ones that, if not delivered reliably, will cause the related machine to behave unexpectedly.

Machines/Apple/Macintosh/Keyboard.cpp

@@ -86,3 +86,13 @@ uint16_t KeyboardMapper::mapped_key_for_key(Inputs::Keyboard::Key key) {
 #undef Bind
 	}
 }
+
+std::set<Inputs::Keyboard::Key> KeyboardMapper::get_essential_modifiers() {
+	// Control is fairly optional for a Mac, but option, meta and shift
+	// are essential.
+	return {
+		Inputs::Keyboard::Key::LeftShift, Inputs::Keyboard::Key::RightShift,
+		Inputs::Keyboard::Key::LeftOption, Inputs::Keyboard::Key::RightOption,
+		Inputs::Keyboard::Key::LeftMeta, Inputs::Keyboard::Key::RightMeta,
+	};
+}

Machines/Apple/Macintosh/Keyboard.hpp

@@ -291,7 +291,8 @@ class Keyboard {
 	Provides a mapping from idiomatic PC keys to Macintosh keys.
 */
 class KeyboardMapper: public KeyboardMachine::MappedMachine::KeyboardMapper {
-	uint16_t mapped_key_for_key(Inputs::Keyboard::Key key) override;
+	uint16_t mapped_key_for_key(Inputs::Keyboard::Key key) final;
+	std::set<Inputs::Keyboard::Key> get_essential_modifiers() final;
 };
 
 }

Machines/KeyboardMachine.hpp

@@ -11,6 +11,7 @@
 
 #include <cstdint>
 #include <string>
+#include <set>
 
 #include "../Inputs/Keyboard.hpp"
 
@@ -65,6 +66,7 @@ class MappedMachine: public Inputs::Keyboard::Delegate, public Machine {
 		class KeyboardMapper {
 			public:
 				virtual uint16_t mapped_key_for_key(Inputs::Keyboard::Key key) = 0;
+				virtual std::set<Inputs::Keyboard::Key> get_essential_modifiers() { return {}; }
 		};
 
 		/// Terminates a key sequence from the character mapper.