Thruster glows rendered too bright

Thruster glows used to be drawn "as-is"; against a black background, you'd get a glow identical to the actual texture being used. Now, they end up drawn much brighter instead.

[ship lab comparison](https://dl.dropboxusercontent.com/u/63964618/fs/thruster_glow_brighter.jpg)

[the test mod used (use GTF Erinyes as in the screenshot)](https://dl.dropboxusercontent.com/u/63964618/fs/thrusterglowtestmod.zip)

As the screenshots show, it used to work right in 3.7.2 and the change happened sometime between it and 3.7.4.