Immutable texture may implemented slightly differently in driver. This
CL adds a new test that calls glGenerateMipmap after changing
GL_BASE_LEVEL and GL_MAX_LEVEL. It also expands PingPongBaseLevel to
test immutable texture as well.

Bug: b/181800403
Test: Texture2DBaseMaxTestES3.PingPongBaseLevelImmutable
Test: Texture2DBaseMaxTestES3.GenerateMipmapAfterRebaseImmutable
Test: Texture2DBaseMaxTestES3.GenerateMipmapAfterRebase
Change-Id: I4ccf58f588bfd5e02a6fc7439b6fe535fca8dd7c
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2787253Reviewed-by: Ian Elliott <ianelliott@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Charlie Lao <cclao@google.com>