We currently don't allow eglMakeCurrent to take an external buffer,
which prevents IOSurfaces from being used as the default framebuffer.
However, all the support for rendering to an IOSurface is in place
already. It should be fine to remove the check against external buffers
in eglMakeCurrent.

Bug chromium:987289

Change-Id: I6342e553085f05914db0193500ea018961256a89
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/34388
Presubmit-Ready: Sean Risser <srisser@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Tested-by: Sean Risser <srisser@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>