twn_input: add twn_scancode.h as port of SDL_scancode.h, remove SDL2/SDL.h header usage sa for plans on having clean public headers

src/twn_input_c.h

@@ -2,6 +2,26 @@
 #define INPUT_INTERNAL_API_H
 
 #include "twn_input.h"
+#include "twn_vec.h"
+
+#include <SDL2/SDL.h>
+
+
+union ButtonCode {
+    SDL_Scancode scancode;
+    SDL_Keycode keycode;
+    SDL_GameControllerButton gamepad_button;
+    uint8_t mouse_button;   /* SDL_BUTTON_ enum */
+};
+
+
+/* an input to which an action is bound */
+/* it is not limited to literal buttons */
+typedef struct Button {
+    enum ButtonSource source;
+    union ButtonCode code;
+} Button;
+
 
 void input_reset_state(InputState *input);