* Align ”right” vector with the horizontal plane

The player's camera in Minetest has limited degrees of freedom: It can
only rotate around the Y axis and look up and down. This means players
can vary pitch and yaw, but never roll. This implies that the ”right”
vector used to construct the lens must be aligned with the horizon.

This constraint was not taken into account before this patch, so the
in-game images rendered by xcam were often somehow tilted. This patch
sets the Y component of the “right” vector to zero to align the “right”
vector with the horizon: This makes the vector determined by its X and Z
components only and thus aligned with any horizontal plane in Minetest's
coordinate system.

init.lua

@@ -110,7 +110,7 @@ local create_photo_pixels = function( player_name )
 	eye.y = eye.y + girl:get_properties().eye_height
 	local dir = girl:get_look_dir()
 	local right = vector.normalize(
-		vector.new( { x=dir.z, y=dir.y, z=-dir.x } )
+		vector.new( { x=dir.z, y=0, z=-dir.x } )
 	)
 	local down = vector.normalize(
 		dir:cross( right )