This patch intends to implement FramebufferTextureEXT on OpenGL
back-ends.
1. Support layered framebuffer attachments.
2. Add new framebuffer completeness rules on layered framebuffer
   attachments.
3. Support FRAMEBUFFER_ATTACHMENT_LAYERED_EXT as a valid <pname>
   parameter of GetFramebufferAttachmentParameteriv.

Note that for an entire level of a cube map:
1. It has no TextureTarget because TEXTURE_CUBE is not a valid
   target for TexImage*D.
2. It corresponds to 6 ImageDescs (that represents its faces) in
   class Texture, so when the cube map is cube complete, we return
   the ImageDesc of its first face, meanwhile we do not allow
   querying ImageDesc if it is not cube complete.

BUG=angleproject:1941
TEST=angle_end2end_tests
     dEQP-GLES31.functional.geometry_shading.query.framebuffer_attachment_layers
     dEQP-GLES31.functional.geometry_shading.query.framebuffer_incomplete_layer_targets
     dEQP-GLES31.functional.geometry_shading.layered.*
     dEQP-GLES31.functional.geometry_shading.instanced.invocation_per_layer_*
     dEQP-GLES31.functional.geometry_shading.instanced.multiple_layers_per_invocation_*

Change-Id: I44393b513ec8f1a682fd1c47d3eaa6f3b3fae877
Reviewed-on: https://chromium-review.googlesource.com/1075811
Commit-Queue: Jiawei Shao <jiawei.shao@intel.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>