Fix rounding in the `pixel_grid_snap` example (#24358)

# Objective

Fixes #24350 

## Solution

I replaced `round` with `floor` as mentioned in the issue.

## Testing

- Did you test these changes? If so, how?

I ran the program on my computer and observed the change.

- Are there any parts that need more testing?

Perhaps other display sizes need testing, but theoretically the math
should be correct. This is my first PR, so I might have missed
something.

- How can other people (reviewers) test your changes? Is there anything
specific they need to know?

Compare the clipping on window sizes that are a multiple of the canvas's
resolution to those that are not.

- If relevant, what platforms did you test these changes on, and are
there any important ones you can't test?

I tested on macOS; I cannot test on any other platform.

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## Showcase

<details>
  <summary>Click to view showcase</summary>

<img width="1728" height="1117" alt="Screenshot 2026-05-19 at 3 14
25 AM"
src="https://github.com/user-attachments/assets/1da0b3f1-1f04-4dbe-bbb2-a14a237ff1a7"
/>
<img width="1728" height="1117" alt="Screenshot 2026-05-19 at 12 50
52 AM"
src="https://github.com/user-attachments/assets/a8fc835e-7b32-49ad-916b-a55c4be1d38f"
/>

</details>

Signed-off-by: Ethan Uppal <113849268+ethanuppal@users.noreply.github.com>