Proposal: kOfxImageClipPropDisplayOffset

Background:
Silhouette (and probably other hosts) starts "time" at frame 0, but has a setting to display time with a frame number offset.
kOfxImageEffectPropFrameRange isn't enough to convey this to plugins, so if a plugin has a custom browser that can display frame time, it doesn't know what this time offset should be.

My proposal is to add a new clip property: **kOfxImageClipPropDisplayOffset**

Type: double X 1
Property Set - an image clip instance (read only)

Normally this would be zero, but on hosts that support it, it would be the human-readable frame offset to be added time values so frame numbers will match the host's user interface.

Start by reviewing the existing kOfxImageEffectPropFrameRange definition and image clip instance property conventions. Check how the proposed read-only double value would be supplied by hosts and consumed by plugins for displayed frame time. Done means the proposal has an agreed design and the OpenFX API specification documents the resulting property and semantics.

Written by the indexing model from the issue text.