egl emulator: fixes for covegl conformance test

Support for eglChooseConfig with configs==NULL.
Also added small change to query EGL_LARGEST_PBUFFER attrib of surfaces.
Fixed error print in QemuPipeStream::readFully.

Those changes make sure that covegl completes & passes on the emulator with Translator
on linux in the backend.

Change-Id: I892917ec102ae85d56a16a7cb321e8c12e7083f1
4 files changed