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/+/1690688Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Commit-Queue: Tim Van Patten <timvp@google.com>