Vulkan: Implement framebuffers without attachments It is possible to render to a framebuffer object that has no attachments. However, the rasterization of primitives is always based on the area and characteristics of the bound framebuffer. These characteristics (size, number of samples, etc.) would normally be defined by the attached images. If no images are attached, these characteristics are defined by their default values. Bug: angleproject:3579 Test: dEQP-GLES31.functional.fbo.*no_attachments* Test: dEQP-GLES31.functional.state_query.framebuffer_default.* Change-Id: I9580b924ac810db573cd8df96273fbb01bbb1f73 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1690688 Reviewed-by: Tim Van Patten <timvp@google.com> Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com> Commit-Queue: Tim Van Patten <timvp@google.com>