FR: Vertex Color & Vertex Alpha support on textured pofs

This is a big request and will require the following:

- a new pof chunk to support BOTH textures and vertex color/alpha
- pof tools support to read vertex color/alpha data
- likely more than my non-programmer brain knows

Presently as per the FSO BSP [data struct](https://wiki.hard-light.net/index.php/BSP_data_structure), only deprecated, untextured FLATPOLY supports vertex color, and does not support vertex alpha at all. For the purposes of this request, black could and likely should be used as vertex alpha.

Vertex alpha (or color w/ black as alpha) support will allow for the complex modeled skyboxes using tile textures to produce parallax effects such as [this skybox](https://cdn.discordapp.com/attachments/430823525016928267/1087164076527992912/unknown.jpg) (sphere, really), from freelancer.

The white nebula effect stretching across the screen uses a tiled texture on a thin band of geometry with vertex alpha fading it out as it stretches out into space. Many such tiled and modeled solutions are possible with the use of vertex alpha, providing a less expensive solution than traditional hi-res cube maps, ease of creation, and modification for parallax effects across systems (eg, a nebula appearing closer or further away between missions)

Vertex color support has two major applications I can think of, with one such simply being the creation of stylized assets using only vertex color, as depicted in [this guide](https://www.indiedb.com/games/oberons-court/news/making-textureless-3d-work-a-how-to-guide).

A more practical application for most modders however is likely the elimination of banding from gradients with a lot of color depth. For example, here is a screenshot of a horizon skybox attempting to make use of a full color texture, vs the same texture in grayscale with vertex color applied:

[full color bitmap](https://cdn.discordapp.com/attachments/407577428731363349/612009393013260308/unknown.png)
[grayscale bitmap](https://gyazo.com/ae2db1ced5a40621332e2c243e741139)
[grayscale bitmap using vertex color](https://i.gyazo.com/thumb/1200/e3a413b4d6dc911c08e8ab99ef4c2326-png.jpg)

Imagine how much better Blue Planet's [Neptune](https://prnt.sc/H1appDwG0Dt0) could look as a grayscale bitmap with vertex color applied!

This is a big request and something I've waited a long time for, and I'm really hoping the scp team is willing to help make it happen.