Fix #1614: add Body:getLocalPoints

[qaisjp]

This is identical to Body:getWorldPoints, except it gets the local points instead.

Fixes #1614.

I've tested this by getting 4 points in each corner around the cursor, converting those points to local points, converting those local points back into world points, and checking that the points are equal. Then using those points to draw a polygon (a square) around the cursor.

Lua script

local world, body

function love.load()
    love.graphics.setBackgroundColor(0, 255, 255)

    love.physics.setMeter(64)

    world = love.physics.newWorld()

    local width, height = love.window.getMode()
    body = love.physics.newBody(world, width/2, height/2)
end

function love.update(dt)
    world:update(dt)
end

function love.draw()
    love.graphics.setLineWidth(12)

    -- Build coordinates for squares around the cursor
    local mx, my = love.mouse.getPosition()
    local points = {
        mx - 30, my - 30, -- top left
        mx + 30, my - 30, -- top right
        mx + 30, my + 30, -- bottom right
        mx - 30, my + 30, -- bottom left
    }

    -- Convert those points local to the body
    local localPoints = {body:getLocalPoints(unpack(points))}

    -- Convert them back to the world
    local worldPoints = {body:getWorldPoints(unpack(localPoints))}

    -- Check that they are equal
    for i, original in ipairs(points) do
        local converted = worldPoints[i]
        assert(original == converted, string.format("Expected %f, got %f", original, converted))
    end

    -- Draw a polgyon
    love.graphics.polygon("line", unpack(worldPoints))
end