Rather then letting gst_gl_memory_setup_buffer guess the GL format used
for an eglimage after importing a dmabuf be explicit about it. This
fixes issues where dmabuf import may have used another format then
gst_gl_format_from_video_info would guess on the basis of the available
GL extensions.
In particular on etnaviv the gst_gl_format_from_video_info would
assuming a luminance + alpha GL format is used for YUY2, but the dmabuf
import will always use RG88. Which causes images to end up somewhat pink when
displayed on the screen.
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