vo_dmabuf_wayland: Support for buffer pre-rotation

On Wayland, similarly like on Android, the compositor can let clients know about the output transform, e.g. 90 deg rotation, in order to allow clients to draw buffers pre-rotated. When clients do so, chances are much higher that the compositor can directly "scan-out" the buffer and put it on a hardware plane without needing to composite/redraw, reducing power consumption.

That is because hardware planes usually don't support rotations. Especially 90/270 deg. ones are rarely supported, while 180 deg is more common.

So whenever mpv can easily do rotations, either because it's using GL/VK already or when a post-processing API offers the feature, it would be great if it did and set the the buffer transform hint accordingly (via wl_surface_set_buffer_transform()) so the compositor knows that the buffer is already rotated.

This is technically very similar to the already supported buffer_scale and preferred_scale and should be thus be quite straight forward to implement.

This would mainly benefit mobile devices where displays are often used rotated 90/270 deg.

See also:

• weston-simple-egl example client (MR).
• https://developer.android.com/games/optimize/vulkan-prerotation
• https://developer.android.com/reference/kotlin/android/view/AttachedSurfaceControl.OnBufferTransformHintChangedListener

Start at the vo_dmabuf_wayland implementation and compare its existing buffer_scale and preferred_scale handling with the Wayland wl_surface_set_buffer_transform() API. Trace which rendering paths can provide a pre-rotated buffer, then verify that the compositor receives the matching transform hint and that normal, 90/270-degree, and 180-degree cases remain correct.