WGPU: Basic texture cubes Allows uploading to texture cube faces and sampling from them in shaders using samplerCube. Bug: angleproject:420782526 Change-Id: I45d4370fcc418f39afb225114d13632a78c7c200 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/6593999 Reviewed-by: Liza Burakova <liza@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Matthew Denton <mpdenton@chromium.org>