VSE: prefetching

When enabled prefetching(preview panel>view settings), a pernament running job
is created, that will render frames in the background until the cache is full.
If the cache is not filled fast enough, prefetch job suspends itself
at the last moment and will wait until it has chance to "catch up".
Effectively this will decouple rendering to separate thread, so rendering
itself is a bit faster.
Cache recycling behavior will be changed to "free furthest frame to the left
of playhead if possible, otherwise rightmost frame".

Reviewed By: brecht

Differential Revision: https://developer.blender.org/D5386

release/scripts/startup/bl_ui/space_sequencer.py

@@ -1894,6 +1894,7 @@ class SEQUENCER_PT_view(SequencerButtonsPanel_Output, Panel):
         layout.use_property_decorate = False
 
         st = context.space_data
+        ed = context.scene.sequence_editor
 
         col = layout.column()
         col.prop(st, "display_channel", text="Channel")
@@ -1905,6 +1906,7 @@ class SEQUENCER_PT_view(SequencerButtonsPanel_Output, Panel):
             col.prop(st, "show_separate_color")
 
         col.prop(st, "proxy_render_size")
+        col.prop(ed, "use_prefetch")
 
 
 class SEQUENCER_PT_frame_overlay(SequencerButtonsPanel_Output, Panel):