Currently, it is assumed that if querying the EGL_TEXTURE_FORMAT of a
Wayland buffer succeeds it is an EGLImage. However, this assumption will no
longer hold on upcoming versions of the NVIDIA EGL Wayland driver which
will include support for querying this attribute for EGLStream buffers as
well. Hence, we need to check if buffers are EGLStreams first.

Fixes #488
!477