# Reduce repeated clip-space work in `draw_sprite`

## Summary

`draw_sprite` calls `to_clip` four times for an axis-aligned quad:

```odin
p0 := to_clip(x0_px, y0_px, sw, sh)
p1 := to_clip(x1_px, y0_px, sw, sh)
p2 := to_clip(x1_px, y1_px, sw, sh)
p3 := to_clip(x0_px, y1_px, sw, sh)
```

This repeats the same divisions and converts duplicate x/y coordinates. An
axis-aligned sprite has only two unique x values and two unique y values.

This is a measurable optimization, but it is low priority at the current
128-sprite limit because the absolute saving is small.

## Evidence

The checked-in unoptimized profiles report `engine::to_clip` at 5.33% and 5.51%
self time. Those percentages are inflated by the unoptimized profiling build,
so the change was also measured with `-debug -o:speed`.

A temporary benchmark used the real baked toad metadata, changed sprite
position on every iteration, preallocated the queue, and performed two million
draws per mode over seven trials:

| Mode | Median time per draw |
| --- | ---: |
| Current `draw_sprite` | 25.128 ns |
| Precomputed clip scale and reused coordinates | 21.531 ns |
| Same math plus cached `Frame_Def` | 21.778 ns |

Simplifying the math improved isolated draw time by approximately **14.3%**.
Caching the resolved frame did not provide an additional benefit and should not
be included without new evidence.

At `MAX_SPRITES == 128`, the measured math saving is only about 0.46
microseconds per completely full frame. GPU/driver work dominated the
end-to-end benchmark, so this should follow the profiling and deterministic
benchmark improvements.

Environment:

- Odin `dev-2026-05-nightly:ea5175d`
- Optimized with `-debug -o:speed`
- Linux x86-64

## Proposed change

Compute clip scaling once and construct corners from the unique coordinates:

```odin
sx := 2.0 / f32(app.swapchain_w)
sy := 2.0 / f32(app.swapchain_h)

left   := x0_px * sx - 1
right  := x1_px * sx - 1
top    := 1 - y0_px * sy
bottom := 1 - y1_px * sy

p0 := Vec2{left, top}
p1 := Vec2{right, top}
p2 := Vec2{right, bottom}
p3 := Vec2{left, bottom}
```

Keep `to_clip` for general callers and its existing tests; this change only
specializes quad construction inside `draw_sprite`.

## Acceptance criteria

- Existing sprite geometry, camera, UV, and batching tests pass.
- Add or extend a test that compares all four generated corners against
  `to_clip` for representative viewport and sprite coordinates.
- Flipped and unflipped sprites produce identical vertices to the current code.
- An optimized deterministic benchmark shows at least a 10% improvement in
  isolated `draw_sprite` time under comparable conditions.
- Do not add a per-sprite frame cache as part of this issue.