GL: Fix support for glCopyTextureCHROMIUM with source rectangle textures. 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>