Make new shadow map code compile under OpenGL ES 2.0

Make the new shadow map code (including shaders) compile correctly
under OpenGL ES on a Mali-400 platform.

Fix distinction between definition of SHADOW_MAP (for directional
and beam lights only) and SPOT_LIGHT_SHADOW_MAP (for spot lights only);
do not perform unnecessary calculations for spot light shadows.

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