Scattered issues

- Fix gl debug spam when reading from the scatter buffer
- Cherry pick optimization from last frame visibility to use the staging
  buffer itself to upload the scatter list

Author: Jozufozu

common/src/backend/java/dev/engine_room/flywheel/backend/engine/indirect/StagingBuffer.java

@@ -10,6 +10,7 @@
 import dev.engine_room.flywheel.backend.compile.IndirectPrograms;
 import dev.engine_room.flywheel.backend.gl.GlFence;
 import dev.engine_room.flywheel.backend.gl.buffer.GlBuffer;
+import dev.engine_room.flywheel.backend.gl.buffer.GlBufferUsage;
 import dev.engine_room.flywheel.lib.memory.FlwMemoryTracker;
 import dev.engine_room.flywheel.lib.memory.MemoryBlock;
 import it.unimi.dsi.fastutil.PriorityQueue;
@@ -22,6 +23,8 @@ public class StagingBuffer {
 	private static final int STORAGE_FLAGS = GL45C.GL_MAP_PERSISTENT_BIT | GL45C.GL_MAP_WRITE_BIT | GL45C.GL_CLIENT_STORAGE_BIT;
 	private static final int MAP_FLAGS = GL45C.GL_MAP_PERSISTENT_BIT | GL45C.GL_MAP_WRITE_BIT | GL45C.GL_MAP_FLUSH_EXPLICIT_BIT | GL45C.GL_MAP_INVALIDATE_BUFFER_BIT;
 
+	private static final int SSBO_ALIGNMENT = GL45.glGetInteger(GL45.GL_SHADER_STORAGE_BUFFER_OFFSET_ALIGNMENT);
+
 	private final int vbo;
 	private final long map;
 	private final long capacity;
@@ -30,7 +33,7 @@ public class StagingBuffer {
 	private final OverflowStagingBuffer overflow = new OverflowStagingBuffer();
 	private final TransferList transfers = new TransferList();
 	private final PriorityQueue<FencedRegion> fencedRegions = new ObjectArrayFIFOQueue<>();
-	private final GlBuffer scatterBuffer = new GlBuffer();
+	private final GlBuffer scatterBuffer = new GlBuffer(GlBufferUsage.STREAM_COPY);
 	private final ScatterList scatterList = new ScatterList();
 
 	/**
@@ -252,7 +255,6 @@ private void dispatchComputeCopies() {
 				.bind();
 
 		// These bindings don't change between dstVbos.
-		GL45.glBindBufferBase(GL45C.GL_SHADER_STORAGE_BUFFER, 0, scatterBuffer.handle());
 		GL45.glBindBufferBase(GL45C.GL_SHADER_STORAGE_BUFFER, 1, vbo);
 
 		int dstVbo;
@@ -274,7 +276,25 @@ private void dispatchComputeCopies() {
 	}
 
 	private void dispatchScatter(int dstVbo) {
-		scatterBuffer.upload(scatterList.ptr(), scatterList.usedBytes());
+		var scatterSize = scatterList.usedBytes();
+
+		// If there's enough space in the staging buffer still, lets write the scatter in it directly.
+		long alignedPos = pos + SSBO_ALIGNMENT - 1 - (pos + SSBO_ALIGNMENT - 1) % SSBO_ALIGNMENT;
+
+		long remaining = capacity - alignedPos;
+		if (scatterSize <= remaining && scatterSize <= totalAvailable) {
+			MemoryUtil.memCopy(scatterList.ptr(), map + alignedPos, scatterSize);
+			GL45.glBindBufferRange(GL45C.GL_SHADER_STORAGE_BUFFER, 0, vbo, alignedPos, scatterSize);
+
+			long alignmentCost = alignedPos - pos;
+
+			usedCapacity += scatterSize + alignmentCost;
+			totalAvailable -= scatterSize + alignmentCost;
+			pos += scatterSize + alignmentCost;
+		} else {
+			scatterBuffer.upload(scatterList.ptr(), scatterSize);
+			GL45.glBindBufferBase(GL45C.GL_SHADER_STORAGE_BUFFER, 0, scatterBuffer.handle());
+		}
 
 		GL45.glBindBufferBase(GL45C.GL_SHADER_STORAGE_BUFFER, 2, dstVbo);