The first time the GBM backend's EGL context is made current after
creation, both the read and draw surfaces are set to EGL_NO_SURFACE.
This will set the GL read and draw buffers to GL_NONE in accordance with
the EGL spec.

When a real surface is later made current, however, the spec is arguably
unclear on whether the read and draw buffers should remain set to
GL_NONE or whether they should be restored to the default GL_BACK.  The
Mesa driver does the latter, the NVIDIA driver does the former.

To work around this difference, Kwin has an explicit call to
glDrawBuffer in GbmSurface::makeContextCurrent. It does not have a
corresponding call to glReadBuffer, though, which can cause some desktop
effects such as background contrast to render incorrectly with the
NVIDIA driver. This change adds that missing call.