VSE: Add frame interpolation option to speed effect

Do cross transition from current to next frame instead of displaying
one image for n frames.

Reviewed By: ISS, sergey, campbellbarton

Differential Revision: https://developer.blender.org/D7417

release/scripts/startup/bl_ui/space_sequencer.py

@@ -1139,6 +1139,8 @@ class SEQUENCER_PT_effect(SequencerButtonsPanel, Panel):
         col = layout.column(align=True)
         if strip_type == 'SPEED':
             col.prop(strip, "multiply_speed")
+            col.prop(strip, "frame_interpolation_mode")
+
         elif strip_type in {'CROSS', 'GAMMA_CROSS', 'WIPE', 'ALPHA_OVER', 'ALPHA_UNDER', 'OVER_DROP'}:
             col.prop(strip, "use_default_fade", text="Default fade")
             if not strip.use_default_fade: