Round the icon's drawing rect when drawing the button

[Rindbee]

Each component of the icon's drawing rect needs to be snapped to an integer.

Fix #88165.

[Maran23]

Looks good to me.
I can confirm that this fixes the issue.
Also, this looks good code wise.
The code before also floored and rounded the icon size, and the rationale here makes sense to me.

[Calinou]

Tested locally, it works as expected. This makes sense to do regardless of the Snap Controls to Pixels project setting, as that project setting should only control snapping of controls as a whole, not their individual draw calls within.

[akien-mga]

Thanks!