Offload terrain fog to vertex shader

[rreusser]

This PR offloads whatever computation can be offloaded to the vertex shader when rendering fog on terrain. This amounts to maybe half of the fog operations. I also compressed a couple uniforms in to vec4, like [fog.r, fog.g, fog.b, fog.opacity] instead of passing the opacity separately.

This may also apply for circles and possibly symbols, but I have not yet addressed that case.

The difference is only visually apparent when the fog range is very narrow, for exmaple [1.0, 1.01].

vert-fog.mp4

[rreusser]

This is probably the most relevant few lines, the remaining fog computation once everything else has been computed in the vertex shader:

mapbox-gl-js/src/shaders/_prelude_fog.fragment.glsl

Lines 71 to 83 in f5d1a1e

	vec3 fog_apply_from_vert(vec3 color, float fog_opac
	#ifdef FOG_HAZE
	, vec4 haze
	#endif
	) {
	#ifdef FOG_HAZE
	color = srgb_to_linear(color);
	color = mix(color, tonemap(color + haze.rgb), haze.a);
	color = linear_to_srgb(color);
	#endif
	return mix(color, u_fog_color.rgb, fog_opac);
	}