Optimize shader specular lighting calculations

Optimize specular lighting calculation in lighting pass shaders
by moving around dot product calculation.

The reflection vector is now used with OpenGL for specular lighting
calculation (more accurate); for GLES2, use of the half-way vector is
the default (faster) but use of the reflection vector can be configured
at compile time in Makefile.conf.

Signed-off-by: Harm Hanemaaijer <fgenfb@yahoo.com>