Render real window content on the GPU render path

Past clear-color + cursor: a GPU-driven head now renders every
visible window's real content too, via surface_content_elements (the
same generic-over-renderer helper the Pixman path uses, unmodified
against gpu.renderer instead of udev.renderer). Content pushed after
the cursor (so the cursor stays on top), in the same front-to-back
`ids` order the Pixman path's own custom_elements already relies on
for correct occlusion between windows - plain painter's-algorithm
draw order, no separate clip needed since content is window-shaped.

Deliberately the *unrounded* path: no corner masking (that's built
against PixmanRenderer specifically on this backend) and no
decorations (border, titlebar) - a GPU-driven head now shows real
window content, square corners, no chrome. Decorations are the
remaining real gap before this path has parity with the software one.

Per-window geometry/position math (geom from window_anims or
w.geometry, band for a decorated window's titlebar reservation,
content_offset clamped non-negative) mirrors the Pixman path's own
content push exactly, including an earlier double-
subtraction and negative-margin fixes - so a CSD client with a real
shadow margin positions the same way on either render path.

Untested on real GPU-enabled hardware as of this writing: builds,
passes clippy, full test suite green, and matches the existing Pixman
path's geometry logic by inspection, but SRDWM_GPU/general.gpu were
both unset on the machine this was built on - noted honestly in
gpu.rs's own module doc comment, DEFAULTS.md, and
IMPLEMENTATION_STATUS.md.

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