Bug fix - Relocated ambient light contribution in GouraudMaterial

<h3> Reason

The ambient color equation was improperly placed after the alpha calculations. This caused the alpha to be set to 1.0 regardless of the alpha assigned. Oops, copy/paste SNAFU!

<h3> Method

Quick fix: simply moved the ambient color contribution into the `gl_fragColor` calculation.

<h3> Usage

No change