So I was experimenting a little more, added the following options to xorg.conf:
...and added a printf statement after the glFlush() from hello.c. Now I launched the hello program from a terminal and alt-tabbed between the terminal and the hello window. Each time I uncovered the hello window the text I put in the printf statement was printed to the terminal but besides the decoration put on by the window manager the hello window remained completely transparent. After a while the hello program segfaulted and about a half minute later the X server finally locked up.
Option "EXANoComposite" "true" Option "EXANoUploadToScreen" Option "EXANoDownloadFromScreen" Option "MigrationHeuristic" "greedy"