Codechange: Ensure SDLK mappings stay in the expected order. (#12608)

Add a constexpr constructor that ensures at compile-time that the source SDLK range matches the target range.

(cherry picked from commit 90029beb49)

# Conflicts:
#	src/video/sdl2_v.cpp
#	src/video/sdl_v.cpp

src/video/sdl2_v.cpp

@@ -518,18 +518,24 @@ std::vector<int> VideoDriver_SDL_Base::GetListOfMonitorRefreshRates()
 }
 
 struct SDLVkMapping {
-	SDL_Keycode vk_from;
-	byte vk_count;
-	byte map_to;
-	bool unprintable;
+	const SDL_Keycode vk_from;
+	const uint8_t vk_count;
+	const uint8_t map_to;
+	const bool unprintable;
+
+	constexpr SDLVkMapping(SDL_Keycode vk_first, SDL_Keycode vk_last, uint8_t map_first, [[maybe_unused]] uint8_t map_last, bool unprintable)
+		: vk_from(vk_first), vk_count(vk_first - vk_last + 1), map_to(map_first), unprintable(unprintable)
+	{
+		assert((vk_last - vk_first) == (map_last - map_first));
+	}
 };
 
-#define AS(x, z) {x, 0, z, false}
-#define AM(x, y, z, w) {x, (byte)(y - x), z, false}
-#define AS_UP(x, z) {x, 0, z, true}
-#define AM_UP(x, y, z, w) {x, (byte)(y - x), z, true}
+#define AS(x, z) {x, x, z, z, false}
+#define AM(x, y, z, w) {x, y, z, w, false}
+#define AS_UP(x, z) {x, x, z, z, true}
+#define AM_UP(x, y, z, w) {x, y, z, w, true}
 
-static const SDLVkMapping _vk_mapping[] = {
+static constexpr SDLVkMapping _vk_mapping[] = {
 	/* Pageup stuff + up/down */
 	AS_UP(SDLK_PAGEUP,   WKC_PAGEUP),
 	AS_UP(SDLK_PAGEDOWN, WKC_PAGEDOWN),

src/video/sdl_v.cpp

@@ -378,15 +378,21 @@ bool VideoDriver_SDL::ClaimMousePointer()
 }
 
 struct SDLVkMapping {
-	uint16_t vk_from;
-	byte vk_count;
-	byte map_to;
+	const uint16_t vk_from;
+	const uint8_t vk_count;
+	const uint8_t map_to;
+
+	constexpr SDLVkMapping(SDL_Keycode vk_first, SDL_Keycode vk_last, uint8_t map_first, [[maybe_unused]] uint8_t map_last)
+		: vk_from(vk_first), vk_count(vk_first - vk_last + 1), map_to(map_first)
+	{
+		assert((vk_last - vk_first) == (map_last - map_first));
+	}
 };
 
-#define AS(x, z) {x, 0, z}
-#define AM(x, y, z, w) {x, (byte)(y - x), z}
+#define AS(x, z) {x, x, z, z, false}
+#define AM(x, y, z, w) {x, y, z, w, false}
 
-static const SDLVkMapping _vk_mapping[] = {
+static constexpr SDLVkMapping _vk_mapping[] = {
 	/* Pageup stuff + up/down */
 	AM(SDLK_PAGEUP, SDLK_PAGEDOWN, WKC_PAGEUP, WKC_PAGEDOWN),
 	AS(SDLK_UP,     WKC_UP),