Add a new packed enum gl::ImageLayout which consists of the following
enum values:

  GL_LAYOUT_GENERAL_EXT
  GL_LAYOUT_COLOR_ATTACHMENT_EXT
  GL_LAYOUT_DEPTH_STENCIL_ATTACHMENT_EXT
  GL_LAYOUT_DEPTH_STENCIL_READ_ONLY_EXT
  GL_LAYOUT_SHADER_READ_ONLY_EXT
  GL_LAYOUT_TRANSFER_SRC_EXT
  GL_LAYOUT_TRANSFER_DST_EXT
  GL_LAYOUT_DEPTH_READ_ONLY_STENCIL_ATTACHMENT_EXT
  GL_LAYOUT_DEPTH_ATTACHMENT_STENCIL_READ_ONLY_EXT

Bug: angleproject:3289

Change-Id: Idc1615717d54fb1193e2f27c84a24993f6007d7b
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1623810Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Michael Spang <spang@chromium.org>