clients/simple-dmabuf-egl: drop Y_INVERT flag

The motivations for this are:

- Y_INVERT is not used by most real-world clients.
  weston-simple-dmabuf-egl and weston-simple-dmabuf-v4l are one only
  known users. Thus this creates a special case just for these demo
  clients.
- Some compositors (wlroots) have dropped support for DMA-BUF flags,
  so the client no longer runs there.
- Dropping the flag allows compositors to use a KMS hardware plane to
  display the buffer.

It keeps the same axis orientation we had in place where we had the
y-invert flag enabled by default, by doing a reflection about x-axis.

Signed-off-by: Simon Ser <contact@emersion.fr>
Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Closes: #675