Be careful with zstd support in Qt 5.13

[Lisandro Damián Nicanor Pérez Meyer]

Hi!

On Wed, 19 Jun 2019 at 17:02, Fabian Vogt <fabian at ritter-vogt.de> wrote:
>
> Hi,
>
> starting with Qt 5.13, it's possible to build qtbase against libzstd, which
> means rcc uses zstandard instead of zlib as compression algorithm by default.
> This means that resource data can now be compressed in two different ways.
> The existing QResource API only had isCompressed() though, which does not allow
> to differentiate between them. A new method compressionAlgorithm() got
> introduced in 5.13 to return whether zlib or zstd was used for compression,
> but obviously all existing codebases do not use this, but instead assume that
> qUncompress always works if the data is compressed. This assumption now fails
> with 5.13 and those applications are unable to read any data.

I forwarded the issue upstream and Thiago made
<https://codereview.qt-project.org/c/qt/qtbase/+/265834> So it can be
backported to solve this issue.