Premultiplied alpha support is broken since v0.6.4

[baumanj]

It looks like the flag was left out of the transition to the new args parser in a95b3c285db85696999bf7f7eefd051680491d30. The following addition is sufficient to get the code running again:

diff --git a/src/main.rs b/src/main.rs
index fd93c31..6199377 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -75,6 +75,9 @@ fn run() -> Result<(), BoxError> {
         .arg(Arg::with_name("dirty-alpha")
             .long("dirty-alpha")
             .help("Keep RGB data of fully-transparent pixels (makes larger, lower quality files)"))
+        .arg(Arg::with_name("premultiplied-alpha")
+            .long("premultiplied-alpha")
+            .help("Alternative compression mode that lowers quality of semi-transparent colors. Warning: at low qualities it may create black artifacts in transparent areas."))
         .arg(Arg::with_name("color")
             .long("color")
             .default_value("ycbcr")

However, the output it generates is invalid per ISOBMFF (ISO 14496-12:2020) § 8.11.12.1 since it requires "Zero or one" iref boxes and the output generated has 2.

[kornelski]

The dirty-alpha workaround I've developed turned out to work much better than I expected, so I've decided to drop support for premultiplied alpha instead.