You're right, it does seem like a rendering issue regarding glx.
I was ssh'ing with X11 fowarding to the CentOS 7 machine, so my host machine (running Fedora 21) was actually doing the rendering. Trying to run glxgears hanged as well.
I actually tried ssh'ing from a windows desktop (with xming and putty) to this CentOS 7 machine and running glxgears, and it works! So it seems like it is a rendering issue or Xserver issue with my Fedora 21 desktop, not a Qt5 issue...
Thank you for all the help!