OpenGL2: r_cubeMapping doesn't work correctly.

[ghost]

@SmileTheory
Hi, nice to see that you still maintain your new renderer, maybe a good time to report a new issue :)
I thought renderer2 is no longer maintained, so I didn't report this issue until now, the issue is not related to the latest changes:

The issue/bug:
If r_cubeMapping is OFF (r_cubeMapping = 0), the backside of textures with an alpha channel (?) has a 'specular effect'.

How to reproduce the issue:

1. Make sure to start ioquake3 with the second renderer enabled (cl_renderer opengl2).
2. Start q3dm19 (e.g.: open the console and type: \devmap q3dm19).
3. Move to one of the yellow metal pillars.
4. Change r_cubeMapping from 0 to 1, and 1 to 0 and do a vid_restart command after each change.
5. Have a look through the metal pillar into the inside to see the backside of the texture. The inside is very shiny.

Here are two screenshots to see the difference:

This is when r_cubeMapping is set to 0 (OFF):

And this is the correct way, how it should look like. r_cubeMapping is set to 1 (ON):

I'm not sure but I think you can squeeze out a few frames by culling the 'shininess' of the backside.

System:
Windows 8.1 64-bit.
Nvidia 1060 GTX.

[ghost]

Seems to be fixed, well done!
Thanks a lot!