gpui: Prevent texture atlas bleeding at image edges

[18o]

Objective

Fixes #62456.

RenderImage can show a thin border along its edges when linear texture filtering samples outside the image's tile in the texture atlas.

Solution

• Clamp polychrome sprite UV coordinates to the centers of the first and last texels in the atlas tile.
• Apply the same sampling bounds in the Metal, DirectX, and WGPU shaders.
• Ensure cropped image sub-tiles remain at least one texel wide and high so the sampling bounds are always valid.

Testing

• Reproduced the issue on macOS using the example from GPUI: RenderImage leaves visible frame/border on same-color background #62456.
• Confirmed that the visible right and bottom borders disappear with the Metal renderer.
• Ran cargo check against the reproduction project.
• Ran cargo test -p gpui_wgpu shader_is_valid_wgsl; all three shader validation tests passed.
• Ran cargo fmt --package gpui -- --check.
• DirectX runtime rendering was not tested because a Windows environment was not available.

Self-Review Checklist:

• I've reviewed my own diff for quality, security, and reliability
• Unsafe blocks (if any) have justifying comments
• The content adheres to Zed's UI standards
• Tests cover the new/changed behavior — verified manually on Metal, but no pixel-level automated regression test was added
• Performance impact has been considered and is acceptable

Showcase

Before: a thin border is visible along the right and bottom edges of an image matching the background color.

After: the image blends uniformly with the background, with no visible border.

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Release Notes:

• Fixed unwanted borders around images rendered from GPUI texture atlases.