[BUG] wrong image width calculation in ImageVariantsUtility.php

# Bug Report

## Prerequisites

* [X] Can you reproduce the problem on TYPO3 v11.5 LTS
* [X] Can you reproduce the problem on TYPO3 v12.4 LTS
* [X] Can you reproduce the problem on TYPO3 v13 LTS
* [X] Did you [perform a cursory search](https://github.com/issues?q=is%3Aissue+user%3Abenjaminkott+repo%3Abenjaminkott%2Fbootstrap_package)
to see if your bug or enhancement is already reported?

## Description

Unfortunately, there is (still) a calculation error regarding image widths in ImageVariantsUtility.php, which results in incorrect image widths for different sizes.

For example, the [CSS](https://github.com/benjaminkott/bootstrap_package/blob/7b7bb52c12431f44b3d73715054dfdf9139c89e5/Resources/Public/Scss/components/_textpic.scss#L45) states:
```css
.textmedia-item, .textpic-item {
width: calc(50% -(40px / 2));
}
```

Which, for example, leads to an image size of 620px with a container width of 1280px. logical, because $1280 * 0.5 - 40 / 2 = 620$

PHP produces the same result in this case, as the gutters are removed first and then the multiplier is applied.
Mathematically a different way, but in this case the same result: $(1280 - 40) * 0.5 = 620$

However, if a different multiplier is used, such as 25% instead of 50%, the situation is unfortunately different.
CSS calculates: $1280 * 0.25 - 40 / 2 = 300$
And PHP calculates: $(1280 - 40) * 0.25 = 310$

**Ergo:**
If you want to reflect the calculation of image/text widths used in SCSS in PHP code, you would first have to apply the multiplier and then subtract half the gutter.

Start by locating ImageVariantsUtility.php and compare its image-width calculation with the SCSS rule in Resources/Public/Scss/components/_textpic.scss. Check the 50% and 25% examples from the report; done means PHP produces widths consistent with the SCSS calculation, including gutter handling.

Written by the indexing model from the issue text.