When writing out the parameter TGLeglImageOES an additional cast is
needed.

Bug: angleproject:6036
Change-Id: I7470a58a6ae17b0185d0c2e3fe1184b698f3efaf
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2944952Reviewed-by: Cody Northrop <cnorthrop@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Gert Wollny <gert.wollny@collabora.com>