Fix exposure in advanced graphics preferences being limited to 1.5 instead of 4 (#2978)

(#2916)

Author: Rye

indra/newview/skins/default/xui/en/floater_preferences_graphics_advanced.xml

@@ -821,7 +821,7 @@
     layout="topleft"
     left="420"
     min_val="0.5"
-    max_val="1.5"
+    max_val="4.0"
     name="RenderExposure"
     show_text="true"
     top_delta="24"