Fix sky texture scaling wrong with fov changes

src/r_main.h

@@ -26,6 +26,7 @@ extern INT32 centerx, centery;
 
 extern fixed_t centerxfrac, centeryfrac;
 extern fixed_t projection, projectiony;
+extern fixed_t fovtan; // field of view
 
 extern size_t validcount, linecount, loopcount, framecount;
 

src/r_sky.c

@@ -76,5 +76,5 @@ void R_SetupSkyDraw(void)
 void R_SetSkyScale(void)
 {
 	fixed_t difference = vid.fdupx-(vid.dupx<<FRACBITS);
-	skyscale = FixedDiv(FRACUNIT, vid.fdupx+difference);
+	skyscale = FixedDiv(fovtan, vid.fdupx+difference);
 }