Shader generation didn't use the correct sampler type and discarded pixels
outside the [0, 1] texcoord which is almost always true when sampling rectangle
textures.

Added end2end test coverage.

BUG=990368

Change-Id: Ifaa73dfa83467d5bfa32e3795b52033254436a77
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1834582
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>