GIF file metadata retrieval and encoding are not working

[rzumer]

Using a GIF file as input causes the input file size and bit rate to remain at 0, and the duration is set to _I64_MIN. Attempting to encode the file causes an unspecified error.

[rzumer]

Commit 7df1619 solves part of the issue, allowing encoding when a target bit rate or file size is set manually, and disabling trimming. However, due to the duration being set to 0, using the file size target mode will not work and result in a file of the lowest possible file size as determined by the encoder.