twn_input: singleton rework, twn_control.h and fixes

src/twn_engine_context_c.h

@@ -4,6 +4,7 @@
 #include "twn_context.h"
 #include "twn_textures_c.h"
 #include "twn_audio_c.h"
+#include "twn_input_c.h"
 #include "twn_engine_api.h"
 #include "rendering/twn_draw_c.h"
 
@@ -18,6 +19,8 @@ typedef struct EngineContext {
     /* user code facing context */
     Context game;
 
+    InputState input;
+
     /* the program's actual argc and argv */
     int argc;
     char **argv;
@@ -64,6 +67,7 @@ typedef struct EngineContext {
     bool was_successful;
 } EngineContext;
 
+/* TODO: does it need to be marked with TWN_API? */
 TWN_API extern EngineContext ctx;
 
 #endif