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>