TMF: Call postDrawEvent also for transparent events in time graph control

TMF: Call postDrawEvent also for transparent events in time graph control

The color of an event may be transparent, but it may contain data that needs to
be displayed through the postDrawEvent callback, so it should be called before
returning.

Change-Id: Ic672a1aeb781c89b9da2da9670e2b6f4968c3c43
Signed-off-by: Geneviève Bastien <gbastien+lttng@versatic.net>
Reviewed-on: https://git.eclipse.org/r/36702
Reviewed-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>
Tested-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>
Tested-by: Hudson CI