Fix buffer updates going to the wrong cmd buffer if barriers were 0

From what I could see only SSAO & SSIL were affected when they both
call:

int zero[1] = { 0 };
RD::get_singleton()->buffer_update(ssao.importance_map_load_counter, 0,
sizeof(uint32_t), &zero, 0);

int zero[1] = { 0 };
RD::get_singleton()->buffer_update(ssil.importance_map_load_counter, 0,
sizeof(uint32_t), &zero, 0);

Also documented what setup_command_buffer & draw_command_buffer are for.

(cherry picked from commit c9ec1f77272de8a828294d2348c4cb1d4577612c)

drivers/vulkan/rendering_device_vulkan.cpp

@@ -5995,7 +5995,7 @@ Error RenderingDeviceVulkan::buffer_update(RID p_buffer, uint32_t p_offset, uint
 	// No barrier should be needed here.
 	// _buffer_memory_barrier(buffer->buffer, p_offset, p_size, dst_stage_mask, VK_PIPELINE_STAGE_TRANSFER_BIT, dst_access, VK_ACCESS_TRANSFER_WRITE_BIT, true);
 
-	Error err = _buffer_update(buffer, p_offset, (uint8_t *)p_data, p_size, p_post_barrier);
+	Error err = _buffer_update(buffer, p_offset, (uint8_t *)p_data, p_size, true);
 	if (err) {
 		return err;
 	}

drivers/vulkan/rendering_device_vulkan.h

@@ -1015,8 +1015,13 @@ class RenderingDeviceVulkan : public RenderingDevice {
 		List<ComputePipeline> compute_pipelines_to_dispose_of;
 
 		VkCommandPool command_pool = VK_NULL_HANDLE;
-		VkCommandBuffer setup_command_buffer = VK_NULL_HANDLE; // Used at the beginning of every frame for set-up.
-		VkCommandBuffer draw_command_buffer = VK_NULL_HANDLE; // Used at the beginning of every frame for set-up.
+		// Used for filling up newly created buffers with data provided on creation.
+		// Primarily intended to be accessed by worker threads.
+		// Ideally this cmd buffer should use an async transfer queue.
+		VkCommandBuffer setup_command_buffer = VK_NULL_HANDLE;
+		// The main cmd buffer for drawing and compute.
+		// Primarily intended to be used by the main thread to do most stuff.
+		VkCommandBuffer draw_command_buffer = VK_NULL_HANDLE;
 
 		struct Timestamp {
 			String description;

servers/rendering/renderer_rd/effects/ss_effects.cpp

@@ -950,7 +950,7 @@ void SSEffects::screen_space_indirect_lighting(Ref<RenderSceneBuffersRD> p_rende
 
 	RD::get_singleton()->draw_command_end_label(); // SSIL
 
-	RD::get_singleton()->compute_list_end(RD::BARRIER_MASK_NO_BARRIER);
+	RD::get_singleton()->compute_list_end(RD::BARRIER_MASK_TRANSFER); // Zeroing importance_map_load_counter depends on us.
 
 	int zero[1] = { 0 };
 	RD::get_singleton()->buffer_update(ssil.importance_map_load_counter, 0, sizeof(uint32_t), &zero, 0); //no barrier
@@ -1332,7 +1332,7 @@ void SSEffects::generate_ssao(Ref<RenderSceneBuffersRD> p_render_buffers, SSAORe
 		RD::get_singleton()->draw_command_end_label(); // Interleave
 	}
 	RD::get_singleton()->draw_command_end_label(); //SSAO
-	RD::get_singleton()->compute_list_end(RD::BARRIER_MASK_NO_BARRIER); //wait for upcoming transfer
+	RD::get_singleton()->compute_list_end(RD::BARRIER_MASK_TRANSFER); // Zeroing importance_map_load_counter depends on us.
 
 	int zero[1] = { 0 };
 	RD::get_singleton()->buffer_update(ssao.importance_map_load_counter, 0, sizeof(uint32_t), &zero, 0); //no barrier