When depth is set to "ZERO_TO_ONE_EXT" using EXT_clip_control
API, don't transform the z position in shader since it now matches
Vulkan's depth range. Toggle transformPositionToVulkanClipSpace
accordingly.

Bug: angleproject:5471
Tests: dEQP-GLES2.functional.clip_control.depth_mode_zero_to_one
Change-Id: I7e715cb3c98cecf5aaa5cdc3822a3a83bae9c720
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2615864
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>