cli: ability to drop gif frames depending on FPS option

This provides no smoothing, only dropping. This means some stuttering in
some cases, because some frames will be displayed for twice as long, but
it should help match a given target.

Fixes #361

Author: anisse

src/bin/gif_source.rs

@@ -1,12 +1,13 @@
 //! This is for reading GIFs as an input for re-encoding as another GIF
 
-use crate::source::{Fps, Source};
+use crate::source::{Fps, Source, DEFAULT_FPS};
 use crate::{BinResult, SrcPath};
 use gif::Decoder;
 use gifski::Collector;
 use std::io::Read;
 
 pub struct GifDecoder {
+    fps: Option<f32>,
     speed: f32,
     decoder: Decoder<Box<dyn Read>>,
     screen: gif_dispose::Screen,
@@ -27,6 +28,7 @@ impl GifDecoder {
         let screen = gif_dispose::Screen::new_decoder(&decoder);
 
         Ok(Self {
+            fps: fps.fps,
             speed: fps.speed,
             decoder,
             screen,
@@ -39,13 +41,20 @@ impl Source for GifDecoder {
     fn collect(&mut self, c: &mut Collector) -> BinResult<()> {
         let mut idx = 0;
         let mut delay_ts = 0;
+        let skip_frames = self.fps.is_some();
+        let wanted_frame_time = 1. / f64::from(self.fps.unwrap_or(DEFAULT_FPS));
+        let mut wanted_presentation_timestamp = 0.;
         while let Some(frame) = self.decoder.read_next_frame()? {
             self.screen.blit_frame(frame)?;
             let pixels = self.screen.pixels_rgba().map_buf(|b| b.to_owned());
             let presentation_timestamp = f64::from(delay_ts) * (1. / (100. * f64::from(self.speed)));
+            delay_ts += u32::from(frame.delay);
+            if skip_frames && presentation_timestamp < wanted_presentation_timestamp {
+                continue
+            }
+            wanted_presentation_timestamp += wanted_frame_time;
             c.add_frame_rgba(idx, pixels, presentation_timestamp)?;
             idx += 1;
-            delay_ts += u32::from(frame.delay);
         }
         Ok(())
     }

src/bin/gifski.rs

@@ -81,7 +81,8 @@ fn bin_main() -> BinResult<()> {
                                    kept.\nIf video is used, it will be resampled to \
                                    this constant rate by dropping and/or duplicating \
                                    frames.\nDefault is 20 for videos. No effect for \
-                                   GIF or PNG input.")
+                                   PNG input. For GIF input, it will be used to drop \
+                                   frames if present.")
                             .value_parser(value_parser!(f32))
                             .value_name("num"))
                         .arg(Arg::new("fast-forward")