org.eclipse.e4.ui.workbench.renderers.swt.CTabRendering

Disposes also the gc and avoids two references to the temp image.

Change-Id: I195056ee57c715d0758166ae7ddab29f0fcade04
Signed-off-by: Lars Vogel <Lars.Vogel@vogella.com>
Reviewed-on: https://git.eclipse.org/r/c/platform/eclipse.platform.ui/+/185999
Tested-by: Platform Bot <platform-bot@eclipse.org>