GLES2 fix for consistent light ordering with BVH

Due to multi pass approach to lighting in GLES2, in some situations the rendered result can look different if lights are presented in a different order.

The order (aside from directional lights) seems to be simply copied from the culling routine (octree or bvh) which is essentially arbitrary. While octree is usually consistent with order, bvh uses a trickle optimize which may result in lights occurring in different order from frame to frame.

This PR adds an extra layer of sorting on GLES2 lights in order to get some kind of order consistency.

drivers/gles2/rasterizer_scene_gles2.cpp

@@ -899,6 +899,10 @@ RID RasterizerSceneGLES2::light_instance_create(RID p_light) {
 
 	light_instance->light_index = 0xFFFF;
 
+	// an ever increasing counter for each light added,
+	// used for sorting lights for a consistent render
+	light_instance->light_counter = _light_counter++;
+
 	if (!light_instance->light_ptr) {
 		memdelete(light_instance);
 		ERR_FAIL_V_MSG(RID(), "Condition ' !light_instance->light_ptr ' is true.");
@@ -3285,6 +3289,30 @@ void RasterizerSceneGLES2::render_scene(const Transform &p_cam_transform, const
 			index++;
 		}
 
+		// for fog transmission, we want some kind of consistent ordering of lights
+		// add any more conditions here in which we need consistent light ordering
+		// (perhaps we always should have it, but don't know yet)
+		if (env && env->fog_transmit_enabled) {
+
+			struct _LightSort {
+
+				bool operator()(LightInstance *A, LightInstance *B) const {
+					return A->light_counter > B->light_counter;
+				}
+			};
+
+			int num_lights_to_sort = render_light_instance_count - render_directional_lights;
+
+			if (num_lights_to_sort) {
+				SortArray<LightInstance *, _LightSort> sorter;
+				sorter.sort(&render_light_instances[render_directional_lights], num_lights_to_sort);
+				// rejig indices
+				for (int i = render_directional_lights; i < render_light_instance_count; i++) {
+					render_light_instances[i]->light_index = i;
+				}
+			}
+		}
+
 	} else {
 		render_light_instances = NULL;
 		render_directional_lights = 0;
@@ -4086,4 +4114,5 @@ void RasterizerSceneGLES2::finalize() {
 }
 
 RasterizerSceneGLES2::RasterizerSceneGLES2() {
+	_light_counter = 0;
 }

drivers/gles2/rasterizer_scene_gles2.h

@@ -88,6 +88,10 @@ public:
 	uint32_t current_refprobe_index;
 	uint32_t current_shader_index;
 
+private:
+	uint32_t _light_counter;
+
+public:
 	RasterizerStorageGLES2 *storage;
 	struct State {
 
@@ -528,6 +532,10 @@ public:
 
 		Rect2 directional_rect;
 
+		// an ever increasing counter for each light added,
+		// used for sorting lights for a consistent render
+		uint32_t light_counter;
+
 		Set<RID> shadow_atlases; // atlases where this light is registered
 	};