Add border/titlebar decoration rendering to the GPU render path

The GPU (SRDWM_GPU=1/general.gpu) path had real window content but
square corners and no border/titlebar. A prior pass investigated a full
port of the Pixman path's decoration rendering and deliberately did not
attempt it blind, given no working GPU-capable hardware on this machine
to verify a single pixel of it against. Asked directly, twice, to build
it anyway rather than leave it.

Scoped smaller than a full port: border top/bottom strips and the
titlebar bitmap now render, reusing the exact cached MemoryRenderBuffers
the Pixman path already builds (renderer-agnostic pixel buffers,
imported for GlesRenderer the same generic way cursor::render_elements
already does for either renderer). Left out on purpose: occlusion-
fragment clipping against overlapping windows, and the left/right border
side strips plus the drop shadow.

Full workspace build/test/clippy clean. Explicitly not visually
verified - same reason as before, no GPU-capable hardware on this
machine.

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