[Alitleis123]

The panel gets Resolve's look and one simple window, and renders get faster and smoother.

The panel

• Resolve's greys with the site's amber, and a real progress bar.
• One window, top to bottom in the order you use it: the clip, Analyse with its Threshold, the Upscaling and Interpolate switches, Output, then Render. The Settings window is gone.
• What each setting does is behind an (i), not written on the panel.
• The clip card follows the playhead, so there is no button to pick the clip.
• "Drawing in-betweens" is now "Interpolate". Hold pattern, Grainy source and the Auto-update box are gone.
• The status line has room for a finished render's whole message.

Running a render

• The render starts with no console window. Stop ends a run early, and Reset clears one that stalled.
• PNG frames are written on other threads while the GPU draws the next one.
• RIFE is asked only for the middle of each pair of drawings and builds every other frame from those. Near a drawing it returned an exact copy, so a render held each drawing for 2 to 4 extra frames.

Not in this PR

• No version bump and no release. update/latest.json is unchanged, so nobody's install updates from this merge.
• docs/help.html describes the new panel, so it will be ahead of v0.4.0 until v0.5.0 ships.

All 775 tests pass, and the release folders are rebuilt.