Refactor sunlight code

Now using shader static parms system. Added cvars:
r_sunlight: enabled GLSL sunlight shader code (#define's USESUNLIGHT
with uniform vec4 SunlightDir and uniform vec4 SunlightColor)
r_sunlight_color: RGB color for sunlight
r_sunlight_intensity:  SunlightColor alpha component
r_sunlight_dir: SunlightDir.xyz
SunlightDir.a is reserved (set to 0).