clients/simple-egl: Fix angle reset on benchmark interval

Commit 62ab6891 intended to change the angle calculation so that the a time delta since the first frame would be used instead of the absolute time. That was done in order to ensure the angle would always start with the same value, allowing users to differentiate left and right, which again is needed when testing flipped transforms.

However, the benchmark_time variable is unsuitable for that purpose as it gets reset on each benchmark interval, abruptly changing the angle.

Thus introduce a dedicated variable.

Fixes 62ab6891

Signed-off-by: Robert Mader robert.mader@collabora.com