It would make sense to have a bound like
`for<'frame> R::Frame<'frame>: AsGlowFrame<'frame>`. But that appears to
not behave properly due to current limitations of the borrow checker:
https://blog.rust-lang.org/2022/10/28/gats-stabilization.html#implied-static-requirement-from-higher-ranked-trait-bounds
Instead, this makes `glow_frame` and `glow_frame_mut` associated
functions of the `AsGlowRenderer` trait. Then it is pretty
straightforward to make the `RenderElement` implementations generic
using that and `FromGlesError`.
It would make sense to make `Self::Error: FromGlessError` a requirement
of the `AsGlowRenderer` trait, but due to the lack of implied bounds
support, that produces a bunch of errors about missing bounds. If Rustc
improves that eventually, some bounds could be cleaned up a bit:
https://github.com/rust-lang/rust/issues/44491
The default implementation always returned `Kind::Unspecified`. Instead,
we should pass this method through to the wrapped element.
This fixes the use of cursor planes. Now trace logs show Smithay
assigning an element to the cursor plane, and tools like `drm_info` show
the cursor plane with an `FB_ID` attached.
