Allow merging main surfaces regardless of fogNum

These surfaces themselves won't be used to render fog, instead that is done by adding surfaces referencing the same geometry, but using one of `tr.fogEqualShader` or `tr.fogLEShader`.

Author: VReaperV

src/engine/renderer/tr_main.cpp

@@ -1863,7 +1863,9 @@ int R_AddDrawSurf( surfaceType_t *surface, shader_t *shader, int lightmapNum, in
 	drawSurf->surface = surface;
 	drawSurf->shader = shader;
 	drawSurf->bspSurface = bspSurface;
-	drawSurf->fog = fogNum;
+	/* Allow the renderer backend to merge main surfaces that have fog, ignoring the fogNum,
+	as it only matters for the emitted fog surfaces */
+	drawSurf->fog = ( shader == tr.fogEqualShader || shader == tr.fogLEShader ) ? fogNum : 0;
 	drawSurf->portalNum = portalNum;
 
 	int entityNum;