Customizable Crispness menu background. (#777)

* Load an alternative Crispness menu background from a CRISPYBG lump (if present).

* Only minimum size matters, E.G. Heretic-style 64x128 flats should still work.

* Have that tab back.

* * Fix code/declaration mixup.
* Match indentation of surrounding code.

src/doom/m_menu.c

@@ -1392,10 +1392,16 @@ static void M_DrawMouse(void)
 #include "m_background.h"
 static void M_DrawCrispnessBackground(void)
 {
-	const byte *const src = crispness_background;
+	const byte *src = crispness_background;
 	pixel_t *dest;
 	int x, y;
 
+	// [NS] Try to load the background from a lump.
+	int lump = W_CheckNumForName("CRISPYBG");
+	if (lump != -1 && W_LumpLength(lump) >= 64*64)
+	{
+		src = W_CacheLumpNum(lump, PU_STATIC);
+	}
 	dest = I_VideoBuffer;
 
 	for (y = 0; y < SCREENHEIGHT; y++)