txbind(): only change blending modes if BLEND_OFF|BLEND_ON is different

from previous time.  Add TXF_OVER txflags bit.

src/textures.c

@@ -181,7 +181,7 @@ int txbind( Texture *tx, int *enabled )
     }
   }
 
-  if(wanted != *enabled) {
+  if((wanted ^ *enabled) & (BLEND_ON|BLEND_OFF)) {
     if(wanted & BLEND_ON) {
 	glEnable( GL_BLEND );
 	if(tx->flags & TXF_ADD) {

src/textures.h

@@ -55,6 +55,7 @@ extern Texture *AllLoadedTextures;	/* List of em */
 #define	  TXF_ALPHA	  0x20	/* "use alpha channel" (if 1-chan, it's 000A) */
 #define	  TXF_INTENSITY	  0x40	/* synthesize alpha = intensity */
 #define	  TXF_ADD	  0x80	/* synthesize alpha = 0 (so "over" == "add") */
+#define	  TXF_OVER	  0x100	/* always use "over" compositing */
 
 #define	TX_APPLY	451	/* Interpret texture values to... */
 #define	  TXF_MODULATE	  0