Do not throw if there is no "images" child in json file tree.

Change-Id: I0b47ed00fecad84859ba3f1751407696878203fb
libavmedia.so needs to link to librt under Linux

libavmedia.so links to libopencollada2gltf.a, which contains a
reference to clock_gettime(). And for clock_gettime one needs
to link to librt, under Linux, at least.
So add -lrt when linking libavmedia.so.

Change-Id: I28547c6216738b44701baaed7eaf44f5ca97e98a
No need to add -lrt for a static library.

Change-Id: I8104301e9aab6e7c732e97446030c9caf6ffec66
fdo78474:Corruption a:graphicData has no info for the image.

The original file contains two text boxes one contains image and the other contains chart.
Image in text box is not getting imported in LO which leads to no contents inside the a:graphicData tag which is causing the corruption.
Root cause is found in ShapeContextHandler::getShape.
mxChartShapeContext.is() returns true even when mnStartToken is not set as NMSP_dmlChart which is causing the issue.
I have added one more condition to handle this.

Change-Id: I6c567d7618b34c1a24f6809801e4460af6894c67
Reviewed-on: https://gerrit.libreoffice.org/9363
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>