This defers the actual Image initialization until the Image is used
as either a Framebuffer Attachment or OpenGL Texture object. This
will allow us to construct an Image from multiple sub resources, like
when we're initializing a mip chain, or a cube map texture.

Also adds a helper "hasDepthOrStencilBits" function to angle::Format.

Bug: angleproject:2318
Change-Id: Ife861560216581a90fc6da32a583f69886c7daea
Reviewed-on: https://chromium-review.googlesource.com/985202Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>