Textures: Allow compressed texture from PBO Apex Legends is using a pixel unpack buffer to populate a compressed 3D texture. We had an explicit check in validation that prevented it, possibly due to emulated formats. Rather than prevent it altogether, check whether the format is emulated and proceed if not. Also add a test that exposes the problem. Test: PBOCompressedTexture3DTest Bug: b/236478448 Bug: angleproject:7464 Change-Id: I90105a396bf0a465c62d67910aad2819ba7f8831 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3719161 Commit-Queue: Cody Northrop <cnorthrop@google.com> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Charlie Lao <cclao@google.com>