Decrease the default number of SDFGI cascades to 4

This improves rendering performance noticeably, especially when the
camera moves fast.

On a medium-sized test scene on a GTX 1080 in 2560×1440, going
from 6 to cascades saves 0.5 ms of frame time while looking visually
identical (as most of the scene fits within the 4 cascades).

scene/resources/environment.h

@@ -147,7 +147,7 @@ private:
 
 	// SDFGI
 	bool sdfgi_enabled = false;
-	int sdfgi_cascades = 6;
+	int sdfgi_cascades = 4;
 	float sdfgi_min_cell_size = 0.2;
 	SDFGIYScale sdfgi_y_scale = SDFGI_Y_SCALE_DISABLED;
 	bool sdfgi_use_occlusion = false;