Adding basic readPixels. This change adds to methods in ImageHelper to read texture data. It also implements FramebufferWgpu::readPixels to get the parameters and read the texture data from an ImageHelper that's stored in mRenderTargetCache. Bug: angleproject:8653 Change-Id: I349ed8a0ae3d8d0e187c658f3402c4f8cac23eb8 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/5441353 Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Liza Burakova <liza@chromium.org> Reviewed-by: Matthew Denton <mpdenton@chromium.org>