gldownload: DMABuf export is broken on Intel

Submitted by Nicolas Dufresne @ndufresne

Link to original bug (#795518)

Description

It would seem that Mesa/Intel recently enabled titling or frame buffer compression on the exported DMABuf, and that broke this pipeline:

gst-launch-1.0 gltestsrc ! glcolorconvert ! gldownload ! waylandsink

Though, right now, we just create EGLImage from texture, which then export whatever internal format was internally chosen. The way the export method work, is that we get to know the format at exportation time:

https://cgit.freedesktop.org/gstreamer/gst-plugins-base/tree/gst-libs/gst/gl/egl/gsteglimage.c#n516

But right now we pass NULL/NULL for the format/modifiers pair. I propose to read and validate these value for now, and just fail the export if it there is a modifiers, or if the format does not match the expected one.

If we figure-out how to add modifiers support in GStreamer later, we could revisit and enable this again.