make it so that this test works on the old OpenGL backend

For the old OpenGL we use the extension manager to check for Mesa
and return true. Mesa seems to be broken for this feature. On the new
OpenGL2 we should be fine as it is handled in our code.

Change-Id: I2ef97b7acc7977aa6b0e60f5182d4ffc7848ae7e
Merge topic 'edge_flag_test_fix' into master

7ac000fa make it so that this test works on the old OpenGL backend