deferred skybox, fixes to use of ARB_depth_clamp extension usage, have TextureKey explicitly in QuadBatch

src/rendering/twn_draw_c.h

@@ -137,6 +137,7 @@ void create_circle_geometry(Vec2 position,
 
 struct QuadBatch {
     size_t size;             /* how many primitives are in current batch */
+    TextureKey texture_key;
     TextureMode mode;        /* how color should be applied */
     bool constant_colored;   /* whether colored batch is uniformly colored */
     bool repeat;             /* whether repeat is needed */