new icon: libgdx (original, plain, line)

[canaleal]

Double check these details before you open a PR

• [x] PR does not match another non-stale PR currently opened
• [x] PR name matches the format new icon: Icon name (versions separated by comma). More details here
• [x] PR's base is the develop branch.
• [x] Your icons are inside a folder as seen here
• [x] SVG matches the standards laid out here
• [x] A new object is added in the devicon.json file as seen here

This PR closes #1664

Link to prove your SVG is correct and up-to-date.

• Libgdx website
• Libgdx github
• Libgdx official logo

The libgdx original logo contains the full name of the tool. As such, aliases were used for all the wordmark versions with only the original, plain, and line SVGs created.

I went with #e74a45 as the fill color for the plain and line SVGs as it's the main color for the tool.