Vulkan: Add support for FBO with unequal sized attachments 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>