This expands the test to check for validation errors and that multiple
formats of IOSurfaces can be read from / rendered to. It fixes a couple
issues in the implementation of the extension too.

Minor fixes in the EGL_ANGLE_iosurface_client_buffer extension text.

Fix a fragile test that was not setting the texture unit a shader is to
sample from.

BUG=angleproject:1649

Change-Id: Ied2a9bfff95cb3a9a7a59008260899eb2fc55575
Reviewed-on: https://chromium-review.googlesource.com/924477Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>