The included patch from upstream fixes the issue described here:
https://bugreports.qt.io/browse/QTBUG-48321
The backing store of certain widgets was being improperly invalidated,
leading to display bugs in, e.g. VLC.
This patch is included in Qt 5.6, so we should remove it when we
upgrade.
Without this, configure will say:
Checking for openal... no
/tmp/nix-build-qtmultimedia-5.5.0.drv-0/qtmultimedia-opensource-src-5.5.0/qtmultimedia.pro:28:
Variable GST_VERSION is not defined.
Checking for resourcepolicy... no
And there may be application runtime errors like
defaultServiceProvider::requestService(): no service found for - "org.qt-project.qt.camera"
After this fix, configure will say:
Checking for openal... no
Checking for gstreamer... yes
Checking for gstreamer_photography... no
Checking for gstreamer_encodingprofiles... yes
Checking for gstreamer_appsrc... yes
Checking for linux_v4l... yes
Checking for resourcepolicy... no
And the above runtime error will not appear.
This fix is similar to 449b6028 ("qt5.multimedia: fix gstreamer
support."), except with Qt 5.5 we also need to set GST_VERSION.
The runtime paths QT_PLUGIN_PATH, QML_IMPORT_PATH, QML2_IMPORT_PATH, and
XDG_DATA_DIRS did not include the appropriate paths from the current
package being built because addToSearchPath does not add directories
which don't exist.
