Extend GPU rendering to every head, not just the first

Phase 2 of the GPU-rendering plan deliberately targeted a single head
(GpuContext::output: Option<(crtc::Handle, GpuOutput)>) as a narrow
proof that GBM+EGL+DrmCompositor rendering works at all on this
hardware. DrmOutputManager already supports driving several crtcs at
once - initialize_output is a per-crtc call on one shared manager,
the same way anvil drives multiple outputs - so this was purely an
unexercised restriction, not an architectural limit.

GpuContext::output is now GpuContext::outputs: Vec<(crtc::Handle,
GpuOutput)>, and udev/platform.rs calls initialize_output for every
connected head in its own bring-up loop instead of only the first
after that loop finishes. A head this fails for individually (already
logged, not fatal) still just has no entry and falls back to the
existing legacy Pixman path, unchanged from before.

session.rs's VBlank handler and its VT-switch resume path (which
excludes GPU-driven crtcs from the legacy set_crtc reassert loop, a
different device fd that must never issue mode-set commands against a
crtc DrmOutputManager already owns) both now look a crtc up in the
Vec instead of comparing against a single stored one.

render.rs's own render-loop lookup uses direct field access
(gpu.outputs.iter_mut().find(...)) rather than an equivalent
&mut self method: the borrow checker treats a method call as
borrowing all of GpuContext, including gpu.renderer needed a few
lines later for the same head, where direct field access lets it see
the two borrows are disjoint.

Still gated behind SRDWM_GPU=1 (unset by default) and untested on
real multi-monitor hardware with the flag on - this machine has one
display, so the actual multi-head path itself only gets exercised
whenever it's set on hardware that has more than one.

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