Support camera rotation in the demo Gstreamer pipeline

[rmader]

The ashpd-demo is an awesome example for how to use the camera portal, however it currently lacks rotation support which is needed on many mobile devices as their cameras are often mounted rotated and the drivers don't support transparently per-rotating the image. Here's an example from the Pinephone Pro:

On the right side you see the output from https://gitlab.gnome.org/-/snippets/4781, providing a simple example for how to enable it with a Gstreamer element that supports rotate-method=auto. That property doesn't have to be supported by the sink though - other possible elements that could do the job include glvideoflip and videoflip.

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Related:

• https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/issues/284

[bilelmoussaoui]

So what else is needed here other than updating the demo to include the latest paintable sink?