fix(VSlideGroup): handle rounding issue if using browser zoom

[elioschmutz]

Description

fixes #13212

Motivation and Context

fixes #13212

How Has This Been Tested?

visually

Before

Bildschirmvideo.aufnehmen.2021-03-17.um.11.14.14.mov

After

Bildschirmvideo.aufnehmen.2021-03-17.um.11.14.56.mov

Markup:

Visit with chrome and set the Zoom to 110%, then resize the screen:

<template>
  <v-container>
    <v-tabs>
      <v-tab>Item One</v-tab>
      <v-tab>Item Two</v-tab>
      <v-tab>Item Three</v-tab>
    </v-tabs>
  </v-container>
</template>

<script>
  export default {
    data: () => ({
    //
    }),
  }
</script>
<style lang="scss">
.v-tabs {
  padding-left: 14px;
  .v-tabs-bar {
    padding-left: 16px;
  }
}
</style>

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Improvement/refactoring (non-breaking change that doesn't add any features but makes things better)

Checklist:

• The PR title is no longer than 64 characters.
• The PR is submitted to the correct branch (master for bug fixes and documentation updates, dev for new features and backwards compatible changes and next for non-backwards compatible changes).
• My code follows the code style of this project.
• I've added relevant changes to the documentation (applies to new features and breaking changes in core library)