Add sky light, physical atmosphere, auto exposure and bloom to the GL 4.6 renderer

shaders/sky_shoulder.glsl

@@ -0,0 +1,16 @@
+// the skydome vertex colours carry Perez luminance scaled by a fixed exposure
+// (see CSkyDome::SetExposure), so the circumsolar region and the horizon band
+// routinely land well above 1.0. hard-clipping them, as this shader used to do,
+// flattened everything within ~20 degrees of the sun - and most of the lower sky -
+// into one solid white patch. compress the top end instead: below the knee the
+// response is unchanged, above it it rolls off towards SKY_SHOULDER_WHITE, so the
+// brightness gradient around the sun survives and the patch keeps a soft edge.
+const float SKY_SHOULDER_KNEE = 0.25;
+const float SKY_SHOULDER_WHITE = 0.95;
+
+vec3 sky_shoulder(vec3 color)
+{
+	const float range = SKY_SHOULDER_WHITE - SKY_SHOULDER_KNEE;
+	vec3 excess = max(color - vec3(SKY_SHOULDER_KNEE), vec3(0.0));
+	return min(color, vec3(SKY_SHOULDER_KNEE)) + range * (vec3(1.0) - exp(-excess / range));
+}