OpenGLES 3.0 allows FBO with unequal sized attachments. This CL removes
assertion that all attachment must have equal size from vulkan backend,
and uses common intersect area to create VkFramebuffer object.

Bug: b/181800403
Change-Id: Icbb12a26784b184ebd91740855672013f64b889d
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2824760
Commit-Queue: Charlie Lao <cclao@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>