OpenGL2: Add fourth cascade for sun shadows, and adjust sun shadow cvar defaults.

code/renderergl2/tr_bsp.c

@@ -3313,6 +3313,9 @@ void RE_LoadWorldMap( const char *name ) {
 	tr.toneMinAvgMaxLevel[1] = -2.0f;
 	tr.toneMinAvgMaxLevel[2] = 0.0f;
 
+	// reset last cascade sun direction so last shadow cascade is rerendered
+	VectorClear(tr.lastCascadeSunDirection);
+
 	tr.worldMapLoaded = qtrue;
 
 	// load it