Update SkyBoxMesh to be GL_TRIANGLES

GL_QUADS, as a lot of old tutorials use, is deprecated for use in
glDrawElements. Found these numbers on a DirectX tutorial, they work
nicely.
Bind index list to a VBO in the mesh's VAO

TLAs are fun ya? After going many different ways all due to what
tutorials I keep finding online, this path works. Bind an index buffer
and pass "nil" to glDrawElements.
Swap top and bottom images for stevecube
Render an indexed, color-only cube in the scene

Because pretty, and it showed me when I had indexed rendering finally
working.
Fix color layout of the textures I'm using

Will shortly turn this into a query into Freeimage to find out exactly
what the layout is so the system supports multiple layouts.
Add docs to how SkyBoxMesh is processed