glimagesink: make multiview none equivalent to mono

Fixes the internal viewconvert to not scale buffers for output with the
following pipeline:

gltestsrc ! glimagesink

It also fixes overlay composition with a resized output with an OpenGL
upstream:

gltestsrc ! timeoverlay ! glimagesink

Cherry picked from
https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/commit/6e591403a77282a7bef24d5d5c671a6d0d788c16

Change-Id: Ia02f9cf89d78b96e37fe7fdddf2b33c1dcf299fc
1 file changed