Add a custom 2D SkyBox renderer to the game client

[copperpixel]

This PR implements a custom 2D SkyBox renderer in the game client that aims to address issues with Valve's renderer without doing any memory patching.

Most notably, the client renderer does the following:

• Fixes sky faces being visibly culled on high FOVs
• Fixes issues with loading skies that don't provide materials for all faces
• Fallbacks to drawing the error texture on missing faces instead of the hall of mirrors effect (error texture is affected by fog so appearance may differ)
• In SP, it makes the skybox_swapper entity work properly

Before	After

Engine renderer can be toggled with r_skybox_use_new_renderer ConVar.

We've shipped this for a few days now in Team Fortress 2 Classified without any reported issues.

Does this PR close any issues?

• [VISUAL] Skyboxes are culled when looking away with a high enough FOV #305 - It was already closed as not planned, but I presume because the proposed solution involved memory patching.

PR Checklist

• My PR follows all guidelines in the CONTRIBUTING.md file
• My PR targets a develop branch OR targets another branch with a specific goal in mind