When cosmic-workspaces is shown, render only layer-shell surfaces

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.

src/wayland/handlers/screencopy/render.rs

@@ -36,7 +36,7 @@ use crate::{
     backend::render::{
         cursor,
         element::{AsGlowRenderer, CosmicElement, DamageElement, FromGlesError},
-        render_workspace, CursorMode, CLEAR_COLOR,
+        render_workspace, CursorMode, ElementFilter, CLEAR_COLOR,
     },
     shell::{CosmicMappedRenderElement, CosmicSurface, WorkspaceRenderElement},
     state::{BackendData, Common, State},
@@ -287,7 +287,7 @@ pub fn render_workspace_to_buffer(
                 None,
                 handle,
                 cursor_mode,
-                true,
+                ElementFilter::ExcludeWorkspaceOverview,
             )
             .map(|res| res.0)
         } else {
@@ -316,7 +316,7 @@ pub fn render_workspace_to_buffer(
                 None,
                 handle,
                 cursor_mode,
-                true,
+                ElementFilter::ExcludeWorkspaceOverview,
             )
             .map(|res| res.0)
         }