Automatically transform cursor hotspot user asks to flip cursor image

[mgi388]

Objective

• As discussed in CustomCursor::Image could use a custom size option #17276 (comment), we should transform the cursor's hotspot if the user is asking for the image to be flipped.
• This becomes more important when a scale transform option exists. It's harder for users to transform the hotspot themselves when using scale because they'd need to look up the image to get its dimensions. Instead, we let Bevy handle the hotspot transforms and make the hotspot field the "original/source" hotspot.
• Refs CustomCursor::Image could use a custom size option #17276.

Solution

• When the image needs to be transformed, also transform the hotspot. If the image does not need to be transformed (i.e. fast path), no hotspot transformation is applied.

Testing

• Ran the example: cargo run --example custom_cursor_image --features=custom_cursor.
• Add unit tests for the hotspot transform function.
• I also ran the example I have in my bevy_cursor_kit crate, which I think is a good illustration of the reason for this PR.
• In the following videos, there is an arrow pointing up. The button hover event fires as I move the mouse over it.
• When I press Y, the cursor flips.
• In the first video, on bevy@main before this PR, notice how the hotspot is wrong after flipping and no longer hovering the button. The arrow head and hotspot are no longer synced.
• In the second video, on the branch of this PR, notice how the hotspot gets flipped as soon as I press Y and the cursor arrow head is in the correct position on the screen and still hovering the button. Speaking back to the objective listed at the start: The user originally defined the source hotspot for the arrow. Later, they decide they want to flip the cursor vertically: It's nice that Bevy can automatically flip the source hotspot for them at the same time it flips the source image.

First video (main):

before.mov

Second video (this PR):

after.mov

[mgi388]

@eero-lehtinen 🙏

[mgi388]

@BenjaminBrienen looks like cursors / this one's A-Windowing FYI :) https://github.com/bevyengine/bevy/pulls?q=is%3Apr+author%3Amgi388+is%3Aclosed

[eero-lehtinen]

Thanks for the quality PRs 👍