Compute SSAO at half resolution by default

This provides a significant speedup for a small quality loss.

The quality loss is generally more noticeable during a project's
early stages of development (e.g. in level blockouts)
than it is in a finished project.

doc/classes/ProjectSettings.xml

@@ -1643,11 +1643,8 @@
 		<member name="rendering/environment/ssao/fadeout_to" type="float" setter="" getter="" default="300.0">
 			Distance at which the screen-space ambient occlusion is fully faded out. Use this hide ambient occlusion at great distances.
 		</member>
-		<member name="rendering/environment/ssao/half_size" type="bool" setter="" getter="" default="false">
-			If [code]true[/code], screen-space ambient occlusion will be rendered at half size and then upscaled before being added to the scene. This is significantly faster but may miss small details.
-		</member>
-		<member name="rendering/environment/ssao/half_size.mobile" type="bool" setter="" getter="" default="true">
-			Lower-end override for [member rendering/environment/ssao/half_size] on mobile devices, due to performance concerns.
+		<member name="rendering/environment/ssao/half_size" type="bool" setter="" getter="" default="true">
+			If [code]true[/code], screen-space ambient occlusion will be rendered at half size and then upscaled before being added to the scene. This is significantly faster but may miss small details. If [code]false[/code], screen-space ambient occlusion will be rendered at full size.
 		</member>
 		<member name="rendering/environment/ssao/quality" type="int" setter="" getter="" default="2">
 			Sets the quality of the screen-space ambient occlusion effect. Higher values take more samples and so will result in better quality, at the cost of performance. Setting to [code]ULTRA[/code] will use the [member rendering/environment/ssao/adaptive_target] setting.

servers/rendering_server.cpp

@@ -2896,8 +2896,7 @@ RenderingServer::RenderingServer() {
 
 	GLOBAL_DEF("rendering/environment/ssao/quality", 2);
 	ProjectSettings::get_singleton()->set_custom_property_info("rendering/environment/ssao/quality", PropertyInfo(Variant::INT, "rendering/environment/ssao/quality", PROPERTY_HINT_ENUM, "Very Low (Fast),Low (Fast),Medium (Average),High (Slow),Ultra (Custom)"));
-	GLOBAL_DEF("rendering/environment/ssao/half_size", false);
-	GLOBAL_DEF("rendering/environment/ssao/half_size.mobile", true);
+	GLOBAL_DEF("rendering/environment/ssao/half_size", true);
 	GLOBAL_DEF("rendering/environment/ssao/adaptive_target", 0.5);
 	ProjectSettings::get_singleton()->set_custom_property_info("rendering/environment/ssao/adaptive_target", PropertyInfo(Variant::FLOAT, "rendering/environment/ssao/adaptive_target", PROPERTY_HINT_RANGE, "0.0,1.0,0.01"));
 	GLOBAL_DEF("rendering/environment/ssao/blur_passes", 2);