Change default post_processing_texture_bits to 10

Since the legacy GL driver is no longer the default,
we don't run into a situation where an unintended fallback to 8-bit happens.

src/defaultsettings.cpp

@@ -312,7 +312,7 @@ void set_default_settings()
 
 	// Effects
 	settings->setDefault("enable_post_processing", "true");
-	settings->setDefault("post_processing_texture_bits", "16");
+	settings->setDefault("post_processing_texture_bits", "10");
 	settings->setDefault("directional_colored_fog", "true");
 	settings->setDefault("inventory_items_animations", "false");
 	settings->setDefault("mip_map", "false");