If we can't open /dev/dma_heap/system, fall back to using memfd_create.
It does not let us make a 'proper' dmabuf, but it is good enough to
test our handling of linear buffers in various formats.
The code now by default puts all planes into the same fd - like
v4l does, too.
The old behavior of one fd per plane can be enabled via --disjoint.
Also, am --undecorated option has been added so that the window
isn't decorated and all that the renderer has to do is display the
dmabuf.
This is useful when debugging just the dmabuf rendering.
This uses the dma-heap kernel api to create a dma-buf
and use it for a GdkDmabufTexture. It supports a few
formats to test how well GL conversion of YUV works.
The YUV code is adapted from weston tests.
