Vulkan allows rendering to non-swapchain images, so pbuffers can always
be supported on every config.  With this change, if the window system
does not support a configuration, EGL_WINDOW_BIT is removed from the
config instead of dropping the config entirely.

Bug: chromium:1034840
Change-Id: Ib972ed8ddf7660c327123fa83ae0674456cf2a35
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2378921
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: back sept 10 - Jamie Madill <jmadill@chromium.org>