It seems we allocate a new `GlesRenderbuffer` every time we screencopy
to an shm buffer.
We probably should use a more complicated approach to do proper damage
tracking without any unnecessary copies, and re-using the GPU buffer,
but as long as this allocates a buffer the age of that buffer should be
treated as `0`.
Fixes corruption in cosmic-workspaces when shm screencopy is used. (For
instance, when Cosmic is run with software rendering.)
Avoids a little duplication (matching two variants, instead of three
backends).
The behavior, including errors and panics, should be unchanged for now.
Performance should also not be impacted.
This should help for adding llvmpipe rendering without a device node on
the kms backend, or for adding a variant for pixman.
This allows `cosmic-workspaces` to rely on cosmic-comp for rendering the
background, and just have transparency. This should be a more reliable
and performant way of doing things, at least for now.
Instead of adding another opaque bool argument, this defines an
`ElementFilter` enum, which makes calls more readable.
Window surfaces are still included in screencopy, as needed for the
workspace previews.
