Added killicon.AddTexture()

[HaodongMo]

Adds the ability to add non-VMT based killicons by passing killicons directly into the killicon table, making the process for creating killicons much easier, and finally allowing for dynamically generated killicons and .png killicons. One image can now be used to populate spawn icon, select icon, and kill icon, much more easily.

[vega0]

IS it possible to combine GWEN.CreateTextureBorder with surface.DrawTexturedRectUV?

[robotboy655]

This wouldn't allow you to use .png kill icons, as the texture field is a texture ID that is still bound to limitations of this function: https://wiki.facepunch.com/gmod/surface.SetTexture

This PR is closer to what you want: https://github.com/Facepunch/garrysmod/pull/1158