Frame selection

[ku1ik]

Adds --select <SELECTOR> for choosing which part of a recording agg should render.

This supports both animated excerpts and discrete/slideshow-style selections:

agg --select 5..30 demo.cast clip.gif
agg --select 12.5 demo.cast frame.gif
agg --select marker:build..marker:test demo.cast build.gif
agg --select markers demo.cast markers.gif

A selector can be:

• .. (default), POS.., ..POS, POS..POS for time ranges
• POS, POS,POS,... for discrete positions
• markers for all marker positions

POS can be a time (12.5, 1m20s, 1:20), percentage (50%), marker (marker:build, marker:3), or event index (event:100). Selection is resolved on the adjusted output timeline, after idle-time limiting and speed adjustment.

Range selections use the normal animated GIF path: visual dedupe, timestamp normalization, and FPS capping. Discrete selections keep every resolved selected position and assign sequential output timestamps using --last-frame-duration.

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People have asked for a few versions of the same underlying capability: render only the interesting parts of a recording, or render selected moments instead of the whole timeline.

This PR handles that with one selection model:

• Add ability to convert segment of a cast #111 wanted segment conversion; ranges such as 5..30 cover that directly.
• Add options to save PNG and SVG snapshots at markers, override image dimensions, transparent background #103 wanted snapshots at markers; markers and marker:<value> provide the marker-selection part in a way that also works for GIF output today.
• Add option to output frames as PNGs #110 and Add option to render each frame as jpeg #88 explored exporting frames/images; this PR does not add new output formats, but gives future frame/image output work (planned!) a common way to choose which terminal states to export.

This avoids separate one-off options for trimming, marker snapshots, and frame picking, and instead unifies them by using one timeline selection syntax.

[ku1ik]

Released in 1.9.0.