Render the real cursor on the GPU render path, not just a clear color

Past clear-color-only (Phase 2): the GPU-driven head now shows the
same moving cursor the Pixman path renders, via cursor::render_elements
- already generic over the renderer (R: Renderer + ImportAll +
ImportMem), so it works unmodified against gpu.renderer (GlesRenderer)
instead of udev.renderer (PixmanRenderer).

GpuElement (gpu.rs) widened from a bare MemoryRenderBufferRenderElement
to crate::elements::OverlayElement<GlesRenderer> - the same
Surface/Memory/Solid enum the Pixman path's own custom_elements already
uses, needed once the element list stopped always being empty.

Window content and decorations remain a real gap - a GPU-driven head
still shows no windows, just its own clear color and cursor. Untested
on real GPU-enabled hardware this work (SRDWM_GPU unset on this
machine's live session); builds, passes clippy, full test suite green.

0 files changed, 0 insertions, 0 deletions