big rendering overhaul (cleaning and api abstraction)

include/twn_context.h

@@ -23,6 +23,10 @@ typedef struct Context {
     /* TODO: actually set it */
     float frame_duration;
 
+    /* it is disabled by having fog_density approximately equal to zero */
+    float fog_start, fog_end, fog_density;
+    Color fog_color;
+
     /* resolution is set from config and dictates both logical and drawing space, as they're related */
     /* even if scaling is done, game logic should never change over that */
     Vec2 resolution;