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

include/twn_texture_modes.h

@@ -9,6 +9,7 @@ typedef enum TextureMode {
     TEXTURE_MODE_OPAQUE,        /* all pixels are solid */
     TEXTURE_MODE_SEETHROUGH,  	/* some pixels are alpha zero */
     TEXTURE_MODE_GHOSTLY,       /* arbitrary alpha values */
+    TEXTURE_MODE_UNKNOWN = -1,  /* a sentinel */
 } TextureMode;
 
 #endif