Compositor: Full-frame base system

This patch adds the base code needed to make the full-frame system work for both current tiled/per-pixel implementation of operations and full-frame.

Two execution models:
- Tiled: Current implementation. Renders execution groups in tiles from outputs to input. Not all operations are buffered. Runs the tiled/per-pixel implementation.
- FullFrame: All operations are buffered. Fully renders operations from inputs to outputs. Runs full-frame implementation of operations if available otherwise the current tiled/per-pixel. Creates output buffers on first read and free them as soon as all its readers have finished, reducing peak memory usage of complex/long trees. Operations are multi-threaded but do not run in parallel as Tiled (will be done in another patch).

This should allow us to convert operations to full-frame in small steps with the system already working and solve the problem of high memory usage.

FullFrame breaking changes respect Tiled system, mainly:
- Translate, Rotate, Scale, and Transform take effect immediately instead of next buffered operation.
- Any sampling is always done over inputs instead of last buffered operation.

Reviewed By: jbakker

Differential Revision: https://developer.blender.org/D11113

release/scripts/startup/bl_ui/space_node.py

@@ -660,8 +660,12 @@ class NODE_PT_quality(bpy.types.Panel):
 
         snode = context.space_data
         tree = snode.node_tree
+        prefs = bpy.context.preferences
 
         col = layout.column()
+        if prefs.experimental.use_full_frame_compositor:
+            col.prop(tree, "execution_mode")
+
         col.prop(tree, "render_quality", text="Render")
         col.prop(tree, "edit_quality", text="Edit")
         col.prop(tree, "chunk_size")