SVG, PDF and GIF support

[Tom]

libvips from version 8.3+ supports GIF, SVG and PDF image inputs if it's properly linked against required third-party libraries during compilation.

Those libraries are:

• librsvg for SVG processing
• giflib for GIF processing
• libpoppler for PDF processing

It would be nice if the Docker image installs and compiles libvips accordingly in order to support those image formats.

[Marc Bachmann]

I've added giflib and libpoppler in 8.4.1.
I didn't see a warning regarding librsvg. So I didn't know about it and didn't add it.

[Tom]

It would very convenient having SVG support as well...
You can release a new tag as 8.4.1-1 or 8.4.1+1 to keep the native version intact without breaking the semver convention.

[Marc Bachmann]

I directly pushed to master. Docker hub will build it and publish if it (as latest) was successful.
I'll update the tag once that ran through.
8f1a1ec

[Tom]

Cool, thank you! Moreover, I think libvips 8.4 can be considered stable today.