gl-renderer: Get rid of begin fence sync

Output repaint uses a pair of fence syncs to profile GPU execution by
retrieving their timestamps once signalled. While the end timestamp
can be rather inaccurate in some cases (drivers reusing sync objects
from previous command buffers), the begin timestamp is never correct
because fence syncs are signalled on command buffer completion.

Get rid of the begin fence sync and use the EXT_disjoint_timer_query
extension to measure the actual repaint duration and extrapolate the
begin timestamp from the end one.

Fixes #342

Signed-off-by: Loïc Molinari <loic.molinari@gmail.com>