Fix world_to_window (#65)

* Fix "orthographic/camera.lua:795: attempt to index a nil value" from `world_to_window`

* Updated world_to_window camera_id param comment

orthographic/camera.lua

@@ -788,16 +788,20 @@ end
 -- on a specific camera's view and projection
 -- Window coordinates are the non-scaled coordinates provided by action.screen_x
 -- and action.screen_y in on_input()
--- @param camera_id
+-- @param camera_id or nil for the first camera
 -- @param world World coordinates as a vector3
 -- @return window coordinates
 function M.world_to_window(camera_id, world)
-	local view = cameras[camera_id].view or MATRIX4
-	local projection = cameras[camera_id].projection or MATRIX4
-	local screen = M.project(view, projection, vmath.vector3(world))
-	local scale_x = screen.x / (dpi_ratio * DISPLAY_WIDTH / WINDOW_WIDTH)
-	local scale_y = screen.y / (dpi_ratio * DISPLAY_HEIGHT / WINDOW_HEIGHT)
-	return vmath.vector3(scale_x, scale_y, 0)
+    camera_id = camera_id or camera_ids[1]
+    assert(camera_id, "You must provide a camera id")
+    assert(world, "You must provide world coordinates to convert")
+    local camera = cameras[camera_id]
+    local view = camera.view or MATRIX4
+    local projection = camera.projection or MATRIX4
+    local screen = M.project(view, projection, vmath.vector3(world))
+    local scale_x = screen.x / (dpi_ratio * DISPLAY_WIDTH / WINDOW_WIDTH)
+    local scale_y = screen.y / (dpi_ratio * DISPLAY_HEIGHT / WINDOW_HEIGHT)
+    return vmath.vector3(scale_x, scale_y, 0)
 end
 
 --- Translate world coordinates to screen coordinates given a