Expose the cursor's renerable via Cursor::renderable and access it in the compositor itself instead of the scene (#3980)

- Exposed `Cursor::renderable()` that will return the renderable
associated with the cursor, if there is any
- No longer adding the `Cursor::renderable()` to the `Scene`. Instead,
adding directly in the compositor
- Updated the associated `SoftwareCursor` implementation and tests for
this new interface
- Existing hardware implementations return `nullptr` for now (but I will
address this in later work, as I state below!)

The `Cursor` is not logically a part of the `Scene` as it is not a
surface or anything. Hence, it is up to the compositor to decide whether
or not it will display it.

This work isn't done, but this is the minimal reviewable chunk that I
want to get in before tackling the rest of it. Coming next:

1. Add `Cursor::needs_compositing` so that the compositor can rely on
that boolean flag to decide whether or not to add that `Cursor`'s
renderable
2. Implement `Cursor::renderable()` for each hardware cursor
3. Render the cursor in the `BasicScreenShooter` based off whether or
not an `include_cursor` flag is set

Author: AlanGriffiths

include/platform/mir/graphics/cursor.h

@@ -20,6 +20,7 @@
 
 #include "mir/geometry/size.h"
 #include "mir/geometry/point.h"
+#include "mir/graphics/renderable.h"
 
 #include <memory>
 
@@ -28,6 +29,7 @@ namespace mir
 namespace graphics
 {
 class CursorImage;
+class Renderable;
 class Cursor
 {
 public:
@@ -37,6 +39,8 @@ class Cursor
     virtual void move_to(geometry::Point position) = 0;
 
     virtual void scale(float new_scale) = 0;
+
+    virtual auto renderable() -> std::shared_ptr<Renderable> = 0;
 protected:
     Cursor() = default;
     virtual ~Cursor() = default;

include/test/mir/test/doubles/stub_cursor.h

@@ -33,6 +33,7 @@ struct StubCursor : public graphics::Cursor
     void hide() override {}
     void move_to(geometry::Point) override {}
     void scale(float) override {}
+    auto renderable() -> std::shared_ptr<graphics::Renderable> override { return nullptr; }
 };
 
 }

src/miroil/mir_server_hooks.cpp

@@ -78,6 +78,8 @@ struct HiddenCursorWrapper : mg::Cursor
 
     void scale(float new_scale) override { wrapped->scale(new_scale); }
 
+    auto renderable() -> std::shared_ptr<mir::graphics::Renderable> override { return wrapped->renderable(); }
+
 private:
     std::shared_ptr<mg::Cursor> const wrapped;
 };

src/platforms/atomic-kms/server/kms/cursor.cpp

@@ -452,3 +452,8 @@ void mir::graphics::atomic::Cursor::scale(float new_scale)
 
     show(current_cursor_image);
 }
+
+auto mir::graphics::atomic::Cursor::renderable() -> std::shared_ptr<Renderable>
+{
+    return nullptr;
+}

src/platforms/atomic-kms/server/kms/cursor.h

@@ -78,6 +78,8 @@ class Cursor : public graphics::Cursor
 
     void scale(float) override;
 
+    auto renderable() -> std::shared_ptr<Renderable> override;
+
 private:
     enum ForceCursorState { UpdateState, ForceState };
     struct GBMBOWrapper;

src/platforms/gbm-kms/server/kms/cursor.cpp

@@ -452,3 +452,9 @@ void mir::graphics::gbm::Cursor::scale(float new_scale)
 
     show(current_cursor_image);
 }
+
+auto mir::graphics::gbm::Cursor::renderable() -> std::shared_ptr<Renderable>
+{
+    return nullptr;
+}
+

src/platforms/gbm-kms/server/kms/cursor.h

@@ -79,6 +79,8 @@ class Cursor : public graphics::Cursor
 
     void scale(float) override;
 
+    auto renderable() -> std::shared_ptr<Renderable> override;
+
 private:
     enum ForceCursorState { UpdateState, ForceState };
     struct GBMBOWrapper;

src/platforms/wayland/cursor.cpp

@@ -138,3 +138,9 @@ void mir::platform::wayland::Cursor::scale(float new_scale)
 
     show(current_cursor_image);
 }
+
+auto mir::platform::wayland::Cursor::renderable() -> std::shared_ptr<graphics::Renderable>
+{
+    return nullptr;
+}
+

src/platforms/wayland/cursor.h

@@ -50,6 +50,8 @@ class Cursor : public graphics::Cursor
 
     void scale(float) override;
 
+    auto renderable() -> std::shared_ptr<graphics::Renderable> override;
+
 private:
     wl_shm* const shm;
     std::function<void()> const flush_wl;

src/server/compositor/default_configuration.cpp

@@ -82,6 +82,7 @@ mir::DefaultServerConfiguration::the_compositor()
                 the_scene(),
                 the_shell(),
                 the_compositor_report(),
+                the_cursor(),
                 composite_delay,
                 true);
         });