[vulkan] Remove hard-coded shadow bias

It didn't really work all that well and isn't necessary with the
front-face culling. One less op per pixel.

libs/video/renderer/vulkan/shader/lighting_cube.frag

@@ -19,7 +19,7 @@ shadow (uint map_id, uint layer, uint mat_id, vec3 pos, vec3 lpos)
 			 : dir.z <= -adir.z ? 3
 			 : dir.z >=  adir.z ? 2 : 0;
 	vec4 p = shadow_mats[mat_id + ind] * vec4 (pos, 1);
-	p = p / (p.w - 0.5);	//FIXME hard-coded bias
+	p = p / p.w;
 	float depth = p.z;
 	// convert from the quake frame to the cubemap frame
 	dir = dir.yzx * vec3 (-1, 1, 1);

libs/video/renderer/vulkan/shader/lighting_plane.frag

@@ -10,7 +10,7 @@ float
 shadow (uint map_id, uint layer, uint mat_id, vec3 pos, vec3 lpos)
 {
 	vec4 p = shadow_mats[mat_id] * vec4 (pos, 1);
-	p = p / (p.w - 0.5);	//FIXME hard-coded bias
+	p = p / p.w;
 	float depth = p.z;
 	vec2 uv = (p.xy + vec2(1)) / 2;
 	return texture (shadow_map[map_id], vec4 (uv, layer, depth));